Algorithm to determine if array contains n...n+m?

Question

I saw this question on Reddit, and there were no positive solutions presented, and I thought it would be a perfect question to ask here. This was in a thread about interview questions:

Write a method that takes an int array of size m, and returns (True/False) if the array consists of the numbers n...n+m-1, all numbers in that range and only numbers in that range. The array is not guaranteed to be sorted. (For instance, {2,3,4} would return true. {1,3,1} would return false, {1,2,4} would return false.

The problem I had with this one is that my interviewer kept asking me to optimize (faster O(n), less memory, etc), to the point where he claimed you could do it in one pass of the array using a constant amount of memory. Never figured that one out.

Along with your solutions please indicate if they assume that the array contains unique items. Also indicate if your solution assumes the sequence starts at 1. (I've modified the question slightly to allow cases where it goes 2, 3, 4...)

edit: I am now of the opinion that there does not exist a linear in time and constant in space algorithm that handles duplicates. Can anyone verify this?

The duplicate problem boils down to testing to see if the array contains duplicates in O(n) time, O(1) space. If this can be done you can simply test first and if there are no duplicates run the algorithms posted. So can you test for dupes in O(n) time O(1) space?

Answer 1

Under the assumption numbers less than one are not allowed and there are no duplicates, there is a simple summation identity for this - the sum of numbers from 1 to m in increments of 1 is (m * (m + 1)) / 2. You can then sum the array and use this identity.

You can find out if there is a dupe under the above guarantees, plus the guarantee no number is above m or less than n (which can be checked in O(N))

The idea in pseudo-code:
0) Start at N = 0
1) Take the N-th element in the list.
2) If it is not in the right place if the list had been sorted, check where it should be.
3) If the place where it should be already has the same number, you have a dupe - RETURN TRUE
4) Otherwise, swap the numbers (to put the first number in the right place).
5) With the number you just swapped with, is it in the right place?
6) If no, go back to step two.
7) Otherwise, start at step one with N = N + 1. If this would be past the end of the list, you have no dupes.

And, yes, that runs in O(N) although it may look like O(N ^ 2)

Note to everyone (stuff collected from comments)

This solution works under the assumption you can modify the array, then uses in-place Radix sort (which achieves O(N) speed).

Other mathy-solutions have been put forth, but I'm not sure any of them have been proved. There are a bunch of sums that might be useful, but most of them run into a blowup in the number of bits required to represent the sum, which will violate the constant extra space guarantee. I also don't know if any of them are capable of producing a distinct number for a given set of numbers. I think a sum of squares might work, which has a known formula to compute it (see Wolfram's)

New insight (well, more of musings that don't help solve it but are interesting and I'm going to bed):

So, it has been mentioned to maybe use sum + sum of squares. No one knew if this worked or not, and I realized that it only becomes an issue when (x + y) = (n + m), such as the fact 2 + 2 = 1 + 3. Squares also have this issue thanks to Pythagorean triples (so 3^2 + 4^2 + 25^2 == 5^2 + 7^2 + 24^2, and the sum of squares doesn't work). If we use Fermat's last theorem, we know this can't happen for n^3. But we also don't know if there is no x + y + z = n for this (unless we do and I don't know it). So no guarantee this, too, doesn't break - and if we continue down this path we quickly run out of bits.

In my glee, however, I forgot to note that you can break the sum of squares, but in doing so you create a normal sum that isn't valid. I don't think you can do both, but, as has been noted, we don't have a proof either way.

---

I must say, finding counterexamples is sometimes a lot easier than proving things! Consider the following sequences, all of which have a sum of 28 and a sum of squares of 140:

[1, 2, 3, 4, 5, 6, 7]
[1, 1, 4, 5, 5, 6, 6] 
[2, 2, 3, 3, 4, 7, 7]

I could not find any such examples of length 6 or less. If you want an example that has the proper min and max values too, try this one of length 8:

[1, 3, 3, 4, 4, 5, 8, 8]

---

Simpler approach (modifying hazzen's idea):

An integer array of length m contains all the numbers from n to n+m-1 exactly once iff

• every array element is between n and n+m-1
• there are no duplicates

(Reason: there are only m values in the given integer range, so if the array contains m unique values in this range, it must contain every one of them once)

If you are allowed to modify the array, you can check both in one pass through the list with a modified version of hazzen's algorithm idea (there is no need to do any summation):

• For all array indexes i from 0 to m-1 do
  1. If array[i] < n or array[i] >= n+m => RETURN FALSE ("value out of range found")
  2. Calculate j = array[i] - n (this is the 0-based position of array[i] in a sorted array with values from n to n+m-1)
  3. While j is not equal to i
     1. If list[i] is equal to list[j] => RETURN FALSE ("duplicate found")
     2. Swap list[i] with list[j]
     3. Recalculate j = array[i] - n
• RETURN TRUE

I'm not sure if the modification of the original array counts against the maximum allowed additional space of O(1), but if it doesn't this should be the solution the original poster wanted.

Answer 2

def test(a, n, m):
    seen = [False] * m
    for x in a:
        if x < n or x >= n+m:
            return False
        if seen[x-n]:
            return False
        seen[x-n] = True
    return False not in seen

print test([2, 3, 1], 1, 3)
print test([1, 3, 1], 1, 3)
print test([1, 2, 4], 1, 3)

Note that this only makes one pass through the first array, not considering the linear search involved in not in. :)

I also could have used a python set, but I opted for the straightforward solution where the performance characteristics of set need not be considered.

Update: Smashery pointed out that I had misparsed "constant amount of memory" and this solution doesn't actually solve the problem.

Answer 3

note: this comment is based on the original text of the question (it has been corrected since)

If the question is posed exactly as written above (and it is not just a typo) and for array of size n the function should return (True/False) if the array consists of the numbers 1...n+1,

... then the answer will always be false because the array with all the numbers 1...n+1 will be of size n+1 and not n. hence the question can be answered in O(1). :)

Answer 4

It seems like we could check for duplicates by multiplying all the numbers n...n+m together, and then comparing that value to the expected product of a sequence with no duplicates m!/(n-1)! (note that this assumes it is impossible for a sequence to pass both the expected sum test and the expected product test).

So adding to hazzen's pseudo-code, we have:

is_range(int[] nums, int n, int m) {
  sum_to_m := (m * (m + 1)) / 2
  expected_sum := sum_to_m - (n * (n - 1)) / 2
  real_sum := sum(nums)
  expected_product := m! / (n - 1)!
  real_product := product(nums)
  return ((real_sum == expected_sum) && (expected_product == real_product))

EDIT: Here's my solution in Java using the Sum of Squares to check for duplicates. It also handles any range (including negative numbers) by shifting the sequence to start at 1.

// low must be less than high
public boolean isSequence(int[] nums, int low, int high) {
    int shift = 1 - low;
    low += shift;
    high += shift;

    int sum = 0;
    int sumSquares = 0;
    for (int i = 0; i < nums.length; i++) {
        int num = nums[i] + shift;

        if (num < low)
            return false;
        else if (num > high)
            return false;

        sum += num;
        sumSquares += num * num;
    }

    int expectedSum = (high * (high + 1)) / 2;

    if (sum != expectedSum)
        return false;

    int expectedSumSquares = high * (high + 1) * (2 * high + 1) / 6;

    if (sumSquares != expectedSumSquares)
        return false;

    return true;
}

Answer 5

If that was a typo and the question is about all numbers being in range 1...n instead, then:

def try_arr(arr):
    n = len(arr)
    return (not any(x<1 or x>n for x in arr)) and sum(arr)==n*(n+1)/2

$ print try_arr([1,2,3])
True

$ print try_arr([1,3,1])
False

$ print try_arr([1,2,4])
False

Notes:

• I am using the definition from the original version that numbers start from 1. Sure code can be modified to account for starting from another number.

• If the size of the array (n) was known, you could modify this to stream data from e.g., input file, and use almost no memory (1 temp variable inside sum() and 1 variable for the current item taken from the stream)

• any() is new in python 2.5 (but you have alternative ways to express the same thing in earlier versions of python)

• it uses O(n) time O(1) space. (update: i wrote it does account for duplicates, but apparently that is not true as demonstrated by a comment to another answer here).

Answer 6

Why do the other solutions use a summation of every value? I think this is risky, because when you add together O(n) items into one number, you're technically using more than O(1) space.

Simpler method:

Step 1, figure out if there are any duplicates. I'm not sure if this is possible in O(1) space. Anyway, return false if there are duplicates.

Step 2, iterate through the list, keep track of the lowest and highest items.

Step 3, Does (highest - lowest) equal m ? If so, return true.

Answer 7

Fail := False; Sum1 := 0; Sum2 := 0; TSum1 := 0; TSum2 := 0;

For i := 1 to m do
  Begin
    TSum1 := TSum1 + i;
    TSum2 := TSum2 + i * i;
    Item := Array[i] - n;
    If (Item < 0) or (Item >= m) then 
      Fail := True
    Else 
      Begin
        Sum1 := Sum1 + Item;
        Sum2 := Sum2 + Item * Item;
      End;
  End;
Fail := Fail Or (Sum1 <> TSum1) or (Sum2 <> TSum2);

Tired and no compiler but I think this gives O(m) runtime and can't be fooled.

Answer 8

Awhile back I heard about a very clever sorting algorithm from someone who worked for the phone company. They had to sort a massive number of phone numbers. After going through a bunch of different sort strategies, they finally hit on a very elegant solution: they just created a bit array and treated the offset into the bit array as the phone number. They then swept through their database with a single pass, changing the bit for each number to 1. After that, they swept through the bit array once, spitting out the phone numbers for entries that had the bit set high.

Along those lines, I believe that you can use the data in the array itself as a meta data structure to look for duplicates. Worst case, you could have a separate array, but I'm pretty sure you can use the input array if you don't mind a bit of swapping.

I'm going to leave out the n parameter for time being, b/c that just confuses things - adding in an index offset is pretty easy to do.

Consider:

for i = 0 to m
  if (a[a[i]]==a[i]) return false; // we have a duplicate
  while (a[a[i]] > a[i]) swapArrayIndexes(a[i], i)
  sum = sum + a[i]
next

if sum = (n+m-1)*m return true else return false

This isn't O(n) - probably closer to O(n Log n) - but it does provide for constant space and may provide a different vector of attack for the problem.

If we want O(n), then using an array of bytes and some bit operations will provide the duplication check with an extra n/32 bytes of memory used (assuming 32 bit ints, of course).

EDIT: The above algorithm could be improved further by adding the sum check to the inside of the loop, and check for:

if sum > (n+m-1)*m return false

that way it will fail fast.

Answer 9

Why do the other solutions use a summation of every value? I think this is risky, because when you add together O(n) items into one number, you're technically using more than O(1) space.

O(1) indicates constant space which does not change by the number of n. It does not matter if it is 1 or 2 variables as long as it is a constant number. Why are you saying it is more than O(1) space? If you are calculating the sum of n numbers by accumulating it in a temporary variable, you would be using exactly 1 variable anyway.

Commenting in an answer because the system does not allow me to write comments yet.

Update (in reply to comments): in this answer i meant O(1) space wherever "space" or "time" was omitted. The quoted text is a part of an earlier answer to which this is a reply to.

Answer 10

If you want to know the sum of the numbers [n ... n + m - 1] just use this equation.

var sum = m * (m + 2 * n - 1) / 2;

That works for any number, positive or negative, even if n is a decimal.

Answer 11

I don't think I explained myself in my original post well (below the solid line). For an input of [1 2 3 4 5], for example, the algorithm computes the sum:

-1 + 2 - 3 + 4 - 5

which should be equal to

-1^5 * ceil(5/2)

The pseudo-code below shows how vectors that do not begin at 1 are checked. The algorithm handles cases where the input vector is not sorted and/or it contains duplicates.

---

The following algorithm solves the problem by calculating the alternating sums of the vector elements:

-1 + 2 - 3 + 4 - 5 + .... + m = (-1)^m * ceil(m/2)

where ceil rounds up to the closest integer. In other words, odd numbers are subtracted from the running total and even numbers are added to it.

function test(data, m)
    altSum = 0
    n = Inf
    mCheck = -Inf
    for ii = 1:m
    {
        if data(ii) < n
            n = data(ii)
        if data(ii) > mCheck
            mCheck = data(ii)
        altSum = altSum + (-1)^data(ii) * data(ii)
    }
    if ((mCheck-n+1!=m) || (-1)^(n+m-1) * ceil((n+m-1)/2) - ((-1)^(n-1) * ceil((n-1)/2)) != altSum
        return false
    else
        return true

Answer 12

Given this -

Write a method that takes an int array of size m ...

I suppose it is fair to conclude there is an upper limit for m, equal to the value of the largest int (2^32 being typical). In other words, even though m is not specified as an int, the fact that the array can't have duplicates implies there can't be more than the number of values you can form out of 32 bits, which in turn implies m is limited to be an int also.

If such a conclusion is acceptable, then I propose to use a fixed space of (2^33 + 2) * 4 bytes = 34,359,738,376 bytes = 34.4GB to handle all possible cases. (Not counting the space required by the input array and its loop).

Of course, for optimization, I would first take m into account, and allocate only the actual amount needed, (2m+2) * 4 bytes.

If this is acceptable for the O(1) space constraint - for the stated problem - then let me proceed to an algorithmic proposal... :)

Assumptions: array of m ints, positive or negative, none greater than what 4 bytes can hold. Duplicates are handled. First value can be any valid int. Restrict m as above.

First, create an int array of length 2m-1, ary, and provide three int variables: left, diff, and right. Notice that makes 2m+2...

Second, take the first value from the input array and copy it to position m-1 in the new array. Initialize the three variables.

• set ary[m-1] - nthVal // n=0
• set left = diff = right = 0

Third, loop through the remaining values in the input array and do the following for each iteration:

• set diff = nthVal - ary[m-1]
• if (diff > m-1 + right || diff < 1-m + left) return false // out of bounds
• if (ary[m-1+diff] != null) return false // duplicate
• set ary[m-1+diff] = nthVal
• if (diff>left) left = diff // constrains left bound further right
• if (diff<right) right = diff // constrains right bound further left

I decided to put this in code, and it worked.

Here is a working sample using C#:

public class Program
{
    static bool puzzle(int[] inAry)
    {
        var m = inAry.Count();
        var outAry = new int?[2 * m - 1];
        int diff = 0;
        int left = 0;
        int right = 0;
        outAry[m - 1] = inAry[0];
        for (var i = 1; i < m; i += 1)
        {
            diff = inAry[i] - inAry[0];
            if (diff > m - 1 + right || diff < 1 - m + left) return false;
            if (outAry[m - 1 + diff] != null) return false;
            outAry[m - 1 + diff] = inAry[i];
            if (diff > left) left = diff;
            if (diff < right) right = diff;
        }
        return true;
    }

    static void Main(string[] args)
    {
        var inAry = new int[3]{ 2, 3, 4 };
        Console.WriteLine(puzzle(inAry));
        inAry = new int[13] { -3, 5, -1, -2, 9, 8, 2, 3, 0, 6, 4, 7, 1 };
        Console.WriteLine(puzzle(inAry));
        inAry = new int[3] { 21, 31, 41 };
        Console.WriteLine(puzzle(inAry));
        Console.ReadLine();
    }

}

Answer 13

How about using XOR with even and odd numbers separately. Think about bit level not integer value itself.

bool is_same(const int* a, const int* b, int len)
{
    int even_xor = 0; 
    int odd_xor = 0;

    for(int i=0;i<len;++i)
    {
        if(a[i] & 0x01) odd_xor ^= a[i];
        else even_xor ^= a[i];

        if(b[i] & 0x01) odd_xor ^= b[i];
        else even_xor ^= b[i];
    }

    return (even_xor == 0) && (odd_xor == 0);
}

Answer 14

Here's a working solution in O(n)

This is using the pseudocode suggested by Hazzen plus some of my own ideas. It works for negative numbers as well and doesn't require any sum-of-the-squares stuff.

function testArray($nums, $n, $m) {
    // check the sum. PHP offers this array_sum() method, but it's
    // trivial to write your own. O(n) here.
    if (array_sum($nums) != ($m * ($m + 2 * $n - 1) / 2)) {
        return false;    // checksum failed.
    }
    for ($i = 0; $i < $m; ++$i) {
        // check if the number is in the proper range
        if ($nums[$i] < $n || $nums[$i] >= $n + $m) {
            return false;  // value out of range.
        }

        while (($shouldBe = $nums[$i] - $n) != $i) {
            if ($nums[$shouldBe] == $nums[$i]) {
                return false;    // duplicate
            }
            $temp = $nums[$i];
            $nums[$i] = $nums[$shouldBe];
            $nums[$shouldBe] = $temp;
        }
    }
    return true;    // huzzah!
}

var_dump(testArray(array(1, 2, 3, 4, 5), 1, 5));  // true
var_dump(testArray(array(5, 4, 3, 2, 1), 1, 5));  // true
var_dump(testArray(array(6, 4, 3, 2, 0), 1, 5));  // false - out of range
var_dump(testArray(array(5, 5, 3, 2, 1), 1, 5));  // false - checksum fail
var_dump(testArray(array(5, 4, 3, 2, 5), 1, 5));  // false - dupe
var_dump(testArray(array(-2, -1, 0, 1, 2), -2, 5)); // true

Answer 15

ciphwn has it right. It is all to do with statistics. What the question is asking is, in statistical terms, is whether or not the sequence of numbers form a discrete uniform distribution. A discrete uniform distribution is where all values of a finite set of possible values are equally probable. Fortunately there are some useful formulas to determine if a discrete set is uniform. Firstly, to determine the mean of the set (a..b) is (a+b)/2 and the variance is (n.n-1)/12. Next, determine the variance of the given set:

variance = sum [i=1..n] (f(i)-mean).(f(i)-mean)/n

and then compare with the expected variance. This will require two passes over the data, once to determine the mean and again to calculate the variance.

References:

• uniform discrete distribution
• variance

Skizz

Answer 16

By working with a[i] % a.length instead of a[i] you reduce the problem to needing to determine that you've got the numbers 0 to a.length - 1.

Answer 17

Counter-example for XOR algorithm.

(can't post it as a comment)

@popopome

For a = {0, 2, 7, 5,} it return true (means that a is a permutation of the range [0, 4) ), but it must return false in this case (a is obviously is not a permutaton of [0, 4) ).

Another counter example: {0, 0, 1, 3, 5, 6, 6} -- all values are in range but there are duplicates.

I could incorrectly implement popopome's idea (or tests), therefore here is the code:

bool isperm_popopome(int m; int a[m], int m, int  n)
{
  /** O(m) in time (single pass), O(1) in space,
      no restrictions on n,
      no overflow,
      a[] may be readonly
  */
  int even_xor = 0;
  int odd_xor  = 0;

  for (int i = 0; i < m; ++i)
    {
      if (a[i] % 2 == 0) // is even
        even_xor ^= a[i];
      else
        odd_xor ^= a[i];

      const int b = i + n;
      if (b % 2 == 0)    // is even
        even_xor ^= b;
      else
        odd_xor ^= b;
    }

  return (even_xor == 0) && (odd_xor == 0);
}

Answer 18

I don't think you need to use sums at all. Just check the minimum and maximum and check for dupes. Checking for dupes is the harder part, since you don't know n in advance, so you cannot sort in one pass. To work around that, relax the condition on the (edit:destination) array. Instead of requiring it to be sorted, go for a cyclical shift of a sorted sequence, so that the array goes [k,k+1,..., n+m-2, n+m-1,n,n+1, ... ,k-2,k-1] for some k.

With the condition above, you can assume that a[0] is already in the right position, then the right position for an element d is (d-a[0]) mod m, assuming zero-based array indexing. For example with [4,?,?,?] you can expect [4,5,6,7] or [4,1,2,3] or [4,5,6,3] or [4,5,2,3].

Then just scan the array once, putting each element in its calculated position, updating the min and max and checking for clashes. If there are no clashes and max-min=m, then the condition is met, otherwise it is false.

Answer 19

A C version of Kent Fredric's Ruby solution

(to facilitate testing)

Counter-example (for C version): {8, 33, 27, 30, 9, 2, 35, 7, 26, 32, 2, 23, 0, 13, 1, 6, 31, 3, 28, 4, 5, 18, 12, 2, 9, 14, 17, 21, 19, 22, 15, 20, 24, 11, 10, 16, 25}. Here n=0, m=35. This sequence misses 34 and has two 2.

It is an O(m) in time and O(1) in space solution.

Out-of-range values are easily detected in O(n) in time and O(1) in space, therefore tests are concentrated on in-range (means all values are in the valid range [n, n+m)) sequences. Otherwise {1, 34} is a counter example (for C version, sizeof(int)==4, standard binary representation of numbers).

The main difference between C and Ruby version: << operator will rotate values in C due to a finite sizeof(int), but in Ruby numbers will grow to accomodate the result e.g.,

Ruby: 1 << 100 # -> 1267650600228229401496703205376

C: int n = 100; 1 << n // -> 16

In Ruby: check_index ^= 1 << i; is equivalent to check_index.setbit(i). The same effect could be implemented in C++: vector<bool> v(m); v[i] = true;

bool isperm_fredric(int m; int a[m], int m, int n)
{
  /**
     O(m) in time (single pass), O(1) in space,
     no restriction on n,
     ?overflow?
     a[] may be readonly
   */
  int check_index = 0;
  int check_value = 0;

  int min = a[0];
  for (int i = 0; i < m; ++i) {

    check_index ^= 1 << i;
    check_value ^= 1 << (a[i] - n); //

    if (a[i] < min)
      min = a[i];
  }
  check_index <<= min - n; // min and n may differ e.g., 
                           //  {1, 1}: min=1, but n may be 0.
  return check_index == check_value;
}

Values of the above function were tested against the following code:

bool *seen_isperm_trusted  = NULL;
bool isperm_trusted(int m; int a[m], int m, int n)
{
  /** O(m) in time, O(m) in space */

  for (int i = 0; i < m; ++i) // could be memset(s_i_t, 0, m*sizeof(*s_i_t));
    seen_isperm_trusted[i] = false;

  for (int i = 0; i < m; ++i) {

    if (a[i] < n or a[i] >= n + m)
      return false; // out of range

    if (seen_isperm_trusted[a[i]-n])
      return false; // duplicates
    else
      seen_isperm_trusted[a[i]-n] = true;
  }

  return true; // a[] is a permutation of the range: [n, n+m)
}

Input arrays are generated with:

void backtrack(int m; int a[m], int m, int nitems)
{
  /** generate all permutations with repetition for the range [0, m) */
  if (nitems == m) {
    (void)test_array(a, nitems, 0); // {0, 0}, {0, 1}, {1, 0}, {1, 1}
  }
  else for (int i = 0; i < m; ++i) {
      a[nitems] = i;
      backtrack(a, m, nitems + 1);
    }
}

Answer 20

A C version of b3's pseudo-code

(to avoid misinterpretation of the pseudo-code)

Counter example: {1, 1, 2, 4, 6, 7, 7}.

int pow_minus_one(int power)
{
  return (power % 2 == 0) ? 1 : -1;
}

int ceil_half(int n)
{
  return n / 2 + (n % 2);
}

bool isperm_b3_3(int m; int a[m], int m, int n)
{
  /**
     O(m) in time (single pass), O(1) in space,
     doesn't use n
     possible overflow in sum
     a[] may be readonly
   */
  int altsum = 0;
  int mina = INT_MAX;
  int maxa = INT_MIN;

  for (int i = 0; i < m; ++i)
    {
      const int v = a[i] - n + 1; // [n, n+m-1] -> [1, m] to deal with n=0
      if (mina > v)
        mina = v;
      if (maxa < v)
        maxa = v;

      altsum += pow_minus_one(v) * v;
    }
  return ((maxa-mina == m-1)
          and ((pow_minus_one(mina + m-1) * ceil_half(mina + m-1)
                - pow_minus_one(mina-1) * ceil_half(mina-1)) == altsum));
}

Answer 21

In Python:

def ispermutation(iterable, m, n):
    """Whether iterable and the range [n, n+m) have the same elements.

       pre-condition: there are no duplicates in the iterable
    """ 
    for i, elem in enumerate(iterable):
        if not n <= elem < n+m:
            return False

    return i == m-1

print(ispermutation([1, 42], 2, 1)    == False)
print(ispermutation(range(10), 10, 0) == True)
print(ispermutation((2, 1, 3), 3, 1)  == True)
print(ispermutation((2, 1, 3), 3, 0)  == False)
print(ispermutation((2, 1, 3), 4, 1)  == False)
print(ispermutation((2, 1, 3), 2, 1)  == False)

It is O(m) in time and O(1) in space. It does not take into account duplicates.

Alternate solution:

def ispermutation(iterable, m, n): 
    """Same as above.

    pre-condition: assert(len(list(iterable)) == m)
    """
    return all(n <= elem < n+m for elem in iterable)

Answer 22

Assuming you know only the length of the array and you are allowed to modify the array it can be done in O(1) space and O(n) time.

The process has two straightforward steps. 1. "modulo sort" the array. [5,3,2,4] => [4,5,2,3] (O(2n)) 2. Check that each value's neighbor is one higher than itself (modulo) (O(n))

All told you need at most 3 passes through the array.

The modulo sort is the 'tricky' part, but the objective is simple. Take each value in the array and store it at its own address (modulo length). This requires one pass through the array, looping over each location 'evicting' its value by swapping it to its correct location and moving in the value at its destination. If you ever move in a value which is congruent to the value you just evicted, you have a duplicate and can exit early. Worst case, it's O(2n).

The check is a single pass through the array examining each value with it's next highest neighbor. Always O(n).

Combined algorithm is O(n)+O(2n) = O(3n) = O(n)

Pseudocode from my solution:

foreach(values[]) 
  while(values[i] not congruent to i)
    to-be-evicted = values[i]
    evict(values[i])   // swap to its 'proper' location
    if(values[i]%length == to-be-evicted%length)
      return false;  // a 'duplicate' arrived when we evicted that number
  end while
end foreach
foreach(values[])
  if((values[i]+1)%length != values[i+1]%length)
    return false
end foreach

I've included the java code proof of concept below, it's not pretty, but it passes all the unit tests I made for it. I call these a 'StraightArray' because they correspond to the poker hand of a straight (contiguous sequence ignoring suit).

public class StraightArray {    
    static int evict(int[] a, int i) {
     int t = a[i];
     a[i] = a[t%a.length];
     a[t%a.length] = t;
     return t;
    }
    static boolean isStraight(int[] values) {
     for(int i = 0; i < values.length; i++) {
      while(values[i]%values.length != i) {
       int evicted = evict(values, i);
       if(evicted%values.length == values[i]%values.length) {
        return false;
       }
      }
     }
     for(int i = 0; i < values.length-1; i++) {
      int n = (values[i]%values.length)+1;
      int m = values[(i+1)]%values.length;
      if(n != m) {
       return false;
      }
     }
     return true;
    }
}

Answer 23

Any one-pass algorithm requires Omega(n) bits of storage.

Suppose to the contrary that there exists a one-pass algorithm that uses o(n) bits. Because it makes only one pass, it must summarize the first n/2 values in o(n) space. Since there are C(n,n/2) = 2^Theta(n) possible sets of n/2 values drawn from S = {1,...,n}, there exist two distinct sets A and B of n/2 values such that the state of memory is the same after both. If A' = S \ A is the "correct" set of values to complement A, then the algorithm cannot possibly answer correctly for the inputs

A A' - yes

B A' - no

since it cannot distinguish the first case from the second.

Q.E.D.

Answer 24

The Answer from "nickf" dows not work if the array is unsorted var_dump(testArray(array(5, 3, 1, 2, 4), 1, 5)); //gives "duplicates" !!!!

Also your formula to compute sum([n...n+m-1]) looks incorrect.... the correct formula is (m(m+1)/2 - n(n-1)/2)

Answer 25

Linear in time, constant in space solution for int m

/** gcc -std=c99 ... */
#include <assert.h>
#include <iso646.h>  // and, or
#include <limits.h>  // INT_MIN
#include <stdbool.h> // bool
#include <stdlib.h>  // abs()

bool inrange(int m; int a[m], int m, int n)
{
  /** Whether min(a[]) == n and max(a[]) == n+(m-1)
  */
  if (m == 0) return true; // empty range is a valid range

  // check out-of-range values
  int max = INT_MIN, min = INT_MAX;
  for (int i = 0; i < m; ++i) {
    if (min > a[i]) min = a[i];
    if (max < a[i]) max = a[i];
  }
  return (min == n and max == n+(m-1));
}

bool isperm_minus2(int m; int a[m], int m, int n)
{
  /** O(m) in time, O(1) in space (for 'typeof(m) == typeof(*a) == int')

      Whether a[] is a permutation of the range [n, n+m).

      feature: It marks visited items using a sign bit.
  */
  if (not inrange(a, m, n))
    return false; // out of range

  assert((INT_MIN - (INT_MIN - 1)) == 1); // check n == INT_MIN
  for (int *p = a; p != &a[m]; ++p) {
    *p -= (n - 1); // [n, n+m) -> [1, m+1)
    assert(*p > 0);
  }

  // determine: are there duplicates
  bool has_duplicates = false;
  for (int i = 0; i < m; ++i) {
    const int j = abs(a[i]) - 1;
    assert(j >= 0);
    assert(j < m);
    if (a[j] > 0)
      a[j] *= -1; // mark
    else { // already seen
      has_duplicates = true;
      break;
    }
  }

  // restore the array
  for (int *p = a; p != &a[m]; ++p) {
    if (*p < 0) 
      *p *= -1; // unmark
    // [1, m+1) -> [n, n+m)
    *p += (n - 1);        
  }

  return not has_duplicates; // no duplicates? (+ inrange)
}

Answer 26

An array contains N numbers, and you want to determine whether two of the numbers sum to a given number K. For instance, if the input is 8,4, 1,6 and K is 10, the answer is yes (4 and 6). A number may be used twice. Do the following. a. Give an O(N2) algorithm to solve this problem. b. Give an O(N log N) algorithm to solve this problem. (Hint: Sort the items first. After doing so, you can solve the problem in linear time.) c. Code both solutions and compare the running times of your algorithms. 4.

Answer 27

Product of m consecutive numbers is divisible by m! [ m factorial ]

---

so in one pass you can compute the product of the m numbers, also compute m! and see if the product modulo m ! is zero at the end of the pass

I might be missing something but this is what comes to my mind ...

something like this in python

my_list1 = [9,5,8,7,6]

my_list2 = [3,5,4,7]

def consecutive(my_list):

count = 0
prod = fact = 1
for num in my_list:
    prod *= num
    count +=1 
    fact *= count
if not prod % fact: 
    return 1   
else:   
    return 0

print consecutive(my_list1)

print consecutive(my_list2)

---

HotPotato ~$ python m_consecutive.py

1

0

Answer 28

I propose the following:

Choose a finite set of prime numbers P_1,P_2,...,P_K, and compute the occurrences of the elements in the input sequence (minus the minimum) modulo each P_i. The pattern of a valid sequence is known.

For example for a sequence of 17 elements, modulo 2 we must have the profile: [9 8], modulo 3: [6 6 5], modulo 5: [4 4 3 3 3], etc.

Combining the test using several bases we obtain a more and more precise probabilistic test. Since the entries are bounded by the integer size, there exists a finite base providing an exact test. This is similar to probabilistic pseudo primality tests.

S_i is an int array of size P_i, initially filled with 0, i=1..K
M is the length of the input sequence
Mn = INT_MAX
Mx = INT_MIN

for x in the input sequence:
  for i in 1..K: S_i[x % P_i]++  // count occurrences mod Pi
  Mn = min(Mn,x)  // update min
  Mx = max(Mx,x)  // and max

if Mx-Mn != M-1: return False  // Check bounds

for i in 1..K:
  // Check profile mod P_i
  Q = M / P_i
  R = M % P_i
  Check S_i[(Mn+j) % P_i] is Q+1 for j=0..R-1 and Q for j=R..P_i-1
  if this test fails, return False

return True

Answer 29

Any contiguous array [ n, n+1, ..., n+m-1 ] can be mapped on to a 'base' interval [ 0, 1, ..., m ] using the modulo operator. For each i in the interval, there is exactly one i%m in the base interval and vice versa.

Any contiguous array also has a 'span' m (maximum - minimum + 1) equal to it's size.

Using these facts, you can create an "encountered" boolean array of same size containing all falses initially, and while visiting the input array, put their related "encountered" elements to true.

This algorithm is O(n) in space, O(n) in time, and checks for duplicates.

def contiguous( values )
 #initialization
 encountered = Array.new( values.size, false )
 min, max = nil, nil
 visited = 0

 values.each do |v|

  index = v % encountered.size

  if( encountered[ index ] )
   return "duplicates"; 
  end

  encountered[ index ] = true
  min = v if min == nil or v < min
  max = v if max == nil or v > max 
  visited += 1
 end

 if ( max - min + 1 != values.size ) or visited != values.size
  return "hole"
 else
  return "contiguous"
 end

end

tests = [ 
[ false, [ 2,4,5,6 ] ], 
[ false, [ 10,11,13,14 ] ] , 
[ true , [ 20,21,22,23 ] ] , 
[ true , [ 19,20,21,22,23 ] ] ,
[ true , [ 20,21,22,23,24 ] ] ,
[ false, [ 20,21,22,23,24+5 ] ] ,
[ false, [ 2,2,3,4,5 ] ]
]

tests.each do |t|
 result = contiguous( t[1] )
 if( t[0] != ( result == "contiguous" ) )
  puts "Failed Test : " + t[1].to_s + " returned " + result
 end
end

Answer 30

I like Greg Hewgill's idea of Radix sorting. To find duplicates, you can sort in O(N) time given the constraints on the values in this array.

For an in-place O(1) space O(N) time that restores the original ordering of the list, you don't have to do an actual swap on that number; you can just mark it with a flag:

//Java: assumes all numbers in arr > 1
boolean checkArrayConsecutiveRange(int[] arr) {

// find min/max
int min = arr[0]; int max = arr[0]
for (int i=1; i<arr.length; i++) {
    min = (arr[i] < min ? arr[i] : min);
    max = (arr[i] > max ? arr[i] : max);
}
if (max-min != arr.length) return false;

// flag and check
boolean ret = true;
for (int i=0; i<arr.length; i++) {
    int targetI = Math.abs(arr[i])-min;
    if (arr[targetI] < 0) {
        ret = false; 
        break;
    }
    arr[targetI] = -arr[targetI];
}
for (int i=0; i<arr.length; i++) {
    arr[i] = Math.abs(arr[i]);
}

return ret;
}

Storing the flags inside the given array is kind of cheating, and doesn't play well with parallelization. I'm still trying to think of a way to do it without touching the array in O(N) time and O(log N) space. Checking against the sum and against the sum of least squares (arr[i] - arr.length/2.0)^2 feels like it might work. The one defining characteristic we know about a 0...m array with no duplicates is that it's uniformly distributed; we should just check that.

Now if only I could prove it.

I'd like to note that the solution above involving factorial takes O(N) space to store the factorial itself. N! > 2^N, which takes N bytes to store.

Answer 31

Oops! I got caught up in a duplicate question and did not see the already identical solutions here. And I thought I'd finally done something original! Here is a historical archive of when I was slightly more pleased:

---

Well, I have no certainty if this algorithm satisfies all conditions. In fact, I haven't even validated that it works beyond a couple test cases I have tried. Even if my algorithm does have problems, hopefully my approach sparks some solutions.

This algorithm, to my knowledge, works in constant memory and scans the array three times. Perhaps an added bonus is that it works for the full range of integers, if that wasn't part of the original problem.

I am not much of a pseudo-code person, and I really think the code might simply make more sense than words. Here is an implementation I wrote in PHP. Take heed of the comments.

function is_permutation($ints) {

  /* Gather some meta-data. These scans can
     be done simultaneously */
  $lowest = min($ints);
  $length = count($ints);

  $max_index = $length - 1;

  $sort_run_count = 0;

  /* I do not have any proof that running this sort twice
     will always completely sort the array (of course only
     intentionally happening if the array is a permutation) */

  while ($sort_run_count < 2) {

    for ($i = 0; $i < $length; ++$i) {

      $dest_index = $ints[$i] - $lowest;

      if ($i == $dest_index) {
        continue;
      }

      if ($dest_index > $max_index) {
        return false;
      }

      if ($ints[$i] == $ints[$dest_index]) {
        return false;
      }

      $temp = $ints[$dest_index];
      $ints[$dest_index] = $ints[$i];
      $ints[$i] = $temp;

    }

    ++$sort_run_count;

  }

  return true;

}

Answer 32

So there is an algorithm that takes O(n^2) that does not require modifying the input array and takes constant space.

First, assume that you know n and m. This is a linear operation, so it does not add any additional complexity. Next, assume there exists one element equal to n and one element equal to n+m-1 and all the rest are in [n, n+m). Given that, we can reduce the problem to having an array with elements in [0, m).

Now, since we know that the elements are bounded by the size of the array, we can treat each element as a node with a single link to another element; in other words, the array describes a directed graph. In this directed graph, if there are no duplicate elements, every node belongs to a cycle, that is, a node is reachable from itself in m or less steps. If there is a duplicate element, then there exists one node that is not reachable from itself at all.

So, to detect this, you walk the entire array from start to finish and determine if each element returns to itself in <=m steps. If any element is not reachable in <=m steps, then you have a duplicate and can return false. Otherwise, when you finish visiting all elements, you can return true:

for (int start_index= 0; start_index<m; ++start_index)
{
    int steps= 1;
    int current_element_index= arr[start_index];
    while (steps<m+1 && current_element_index!=start_index)
    {
        current_element_index= arr[current_element_index];
        ++steps;
    }

    if (steps>m)
    {
        return false;
    }
}

return true;

You can optimize this by storing additional information:

1. Record sum of the length of the cycle from each element, unless the cycle visits an element before that element, call it sum_of_steps.
2. For every element, only step m-sum_of_steps nodes out. If you don't return to the starting element and you don't visit an element before the starting element, you have found a loop containing duplicate elements and can return false.

This is still O(n^2), e.g. {1, 2, 3, 0, 5, 6, 7, 4}, but it's a little bit faster.

Answer 33

Hazzen's algorithm implementation in C

#include<stdio.h>

#define swapxor(a,i,j) a[i]^=a[j];a[j]^=a[i];a[i]^=a[j];

int check_ntom(int a[], int n, int m) {
    int i = 0, j = 0;
    for(i = 0; i < m; i++) {
        if(a[i] < n || a[i] >= n+m) return 0;   //invalid entry
        j = a[i] - n;
        while(j != i) {
            if(a[i]==a[j]) return -1;           //bucket already occupied. Dupe.
            swapxor(a, i, j);                   //faster bitwise swap
            j = a[i] - n;
            if(a[i]>=n+m) return 0;             //[NEW] invalid entry
        }
    }
    return 200;                                 //OK
}

int main() {
    int n=5, m=5;
    int a[] = {6, 5, 7, 9, 8};
    int r = check_ntom(a, n, m);
    printf("%d", r);
    return 0;
}

Edit: change made to the code to eliminate illegal memory access.