Uva's 3n+1 problem

Question

I'm solving Uva's 3n+1 problem and I don't get why the judge is rejecting my answer. The time limit hasn't been exceeded and the all test cases I've tried have run correctly so far.

   import java.io.*;



public class NewClass{

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws IOException {

        int maxCounter= 0; 
        int input; 

        int lowerBound; 
        int upperBound; 
        int counter;
        int numberOfCycles;
        int maxCycles= 0;
        int lowerInt;
        BufferedReader consoleInput = new BufferedReader(new InputStreamReader(System.in));
        String line = consoleInput.readLine();
        String [] splitted =  line.split(" ");

        lowerBound = Integer.parseInt(splitted[0]);
        upperBound = Integer.parseInt(splitted[1]);


        int [] recentlyused =  new int[1000001];



if (lowerBound > upperBound )
{
    int h = upperBound;
    upperBound = lowerBound;
    lowerBound = h;

}
lowerInt = lowerBound;
        while (lowerBound <= upperBound)
        {
            counter = lowerBound;
            numberOfCycles = 0;


            if (recentlyused[counter] == 0)
            {
                while ( counter != 1 )
                {


                        if (recentlyused[counter] != 0)
                        {

                        numberOfCycles = recentlyused[counter] + numberOfCycles;
                        counter = 1;
                        }
                        else
                        {
                            if (counter % 2 == 0)
                            {
                            counter = counter /2;
                            }
                            else
                            {
                            counter = 3*counter + 1;
                            }
                            numberOfCycles++;
                        }

                }
            }
            else
            {

            numberOfCycles = recentlyused[counter] + numberOfCycles;
            counter = 1;
            }

            recentlyused[lowerBound] = numberOfCycles;



            if (numberOfCycles > maxCycles)
            {
            maxCycles = numberOfCycles;
            }

            lowerBound++;
        }
        System.out.println(lowerInt +" "+ upperBound+ " "+ (maxCycles+1));

    }


}

Answer 1

Are you making sure to accept the entire input? It looks like your program terminates after reading only one line, and then processing one line. You need to be able to accept the entire sample input at once.

Answer 2

If I understand correctly you are using a memoizing approach. You create a table where you store full results for all the elements you have already calculated so that you do not need to re-calculate results that you already know (calculated before).

The approach itself is not wrong, but there are a couple of things you must take into account. First, the input consists of a list of pairs, you are only processing the first pair. Then, you must take care of your memoizing table limits. You are assuming that all numbers you will hit fall in the range [1...1000001), but that is not true. For the input number 999999 (first odd number below the upper limit) the first operation will turn it into 3*n+1, which is way beyond the upper limit of the memoization table.

Some other things you may want to consider are halving the memoization table and only memorize odd numbers, since you can implement the divide by two operation almost free with bit operations (and checking for even-ness is also just one bit operation).