Problem in project euler #32

Question

Hi,

I'm trying to solve problem 32 of project euler in Java but I'm not getting the correct answer. I'm getting the answer of the following program as 48146. Could anyone please tell me what's wrong I'm doing here and how can I solve that problem?

Here's the code through which I'm trying to solve that problem:

public class Euler32
{

   public static boolean checkValue(char c,String s,int j)
   {   

       for(int i=j+1;i<s.length();i++)
     if(c==s.charAt(i)) return true;

  return false;
 } 



 public static void main(String[] args)
 {
long total=0;
long sum=0;

  for(int i1=40;i1<=999;i1++)
  {   
      for(int j=130;j<=9999;j++)
      {

       sum=i1*j;
       String s=i1+""+j+""+sum;

       if(s.length()!=9) continue;


      else
         {
          for(int i=0;i<s.length()-1;i++)
         {


        if(checkValue(s.charAt(i),s,i))
        break;

     if(i+1==s.length()-1) total+=sum;


         }
      } 
        }
    }
 System.out.println("Total sum is: "+total);
  }  



 }

Answer 1

You are missing some points:

1. It is 1..9
2. Unique products
3. How comes you use 40 and 130(just curious)

(remember, understanding the problem solves 50% of it)

Recommendations:

1. Learn about continue [label]; statement and use a local for loop instead of checkValue
2. Why use s.length() more than once?
3. Use StringBuilder

Answer 2

ok, few notes:

1. Your assumption to start from 40 is incorrect, since 4*1783=6952 is pandigital. And there is another one you are missing :)
2. You are not excluding duplicate products.

Based on the discussion with Marius, im updating the original answer, here. This is the isPandigital method only.

private boolean isPandigital(int a,int b){
   int c=a*b;
   StringBuilder st =  new StringBuilder();
   st.append(a).append(b).append(c);

   if (st.length()!=9 || st.indexOf("0")>-1) return false;

   Set<Character> x=new TreeSet<Character>();

   for (int i=0;i<9; i++){
       x.add(st.charAt(i));
   }
  if (x.size()==9){
     for (int k=0;k<=cnt;k++){
        if (products[k]==c) return false;
     }
  products[++cnt]=c;
  total += c;

  return true;
  }
 return false;
}

I compared both codes on my machine, ten tries then average, the results are as follows:

1. Marius code: average (2070ms)
2. medopal code: average (2200ms)
3. Marius code (using String): average (4200ms)
4. medopal code (using String): average (5000ms)

I added the String vs StringBuilder comparison because apperantly it was the thing that made my original code so slow. But using StringBuilder, Marius code beats me in average 200ms only :)

Lessons learned:

• StringBuilder takes almost half the time String does. (in this experiment at least)
• When you are lazy to think, you can use ready structures. (again in this experiment at least), im not generalizing

Thanks Marius :)