Permutation of String letters: How to remove repeated permutations?

Question

Here is a standard function to print the permutations of characters of a string:

void permute(char *a, int i, int n)
{
   int j;
   if (i == n)
     printf(\         


        

Answer 1

Take notes which chars you swapped previously:

 char was[256];
 /*
 for(j = 0; j <= 255; j++)
    was[j] = 0;
 */
 bzero(was, 256);
 for (j = i; j <= n; j++)
 {
    if (!was[*(a+j)]) {
      swap((a+i), (a+j));
      permute(a, i+1, n);
      swap((a+i), (a+j)); //backtrack
      was[*(a+j)] = 1;
    }
 }

This has to be the fastest one from the entries so far, some benchmark on a "AAAABBBCCD" (100 loops):

native C             - real    0m0.547s
STL next_permutation - real    0m2.141s