Sorting provides a alternative solution: sort the array, and then traverse a linear path over pairs of indices from (0,n-1) to (n-1,0) whose sum straddles the target:

    int main() {
      /* read in and sort array a */

      int i=0,j=N-1, sum;

      while ((sum = a[i] + a[j]) != tgt) {
        if (sum < tgt)
          i++;
        else
          j--;
      }

      printf("%d %d\n", i, j);
    }