Improved Permutations with the BigInteger Data Type: Listing 2

The Permutation Successor method.

public Permutation Successor()
{
  if ((left == 0) && data[left] > data[left+1)) 
       return null;
  Permutation result = new Permutation(this.n);

  for (int idx = 0; idx < result.n; ++idx)  // curr data to result
    result.data[idx] = this.data[idx];
 
  int left, right;

  left = n - 2;  // Find left value 
  while ((result.data[left] > result.data[left + 1]) && (left >= 1))
    --left;

  right = n - 1;  // find right; first value > left
  while (result.data[left] > result.data[right])
    --right;

  int tmp = result.data[left];  // swap [left] and [right]
  result.data[left] = result.data[right];
  result.data[right] = tmp;

  int i = left + 1;  // order the tail
  int j = n - 1;
  while (i < j)
  {
    tmp = result.data[i];
    result.data[i++] = result.data[j];
    result.data[j--] = tmp;
  }

  return result;
}

About the Author

Dr. James McCaffrey works for Microsoft Research in Redmond, Wash. He has worked on several Microsoft products including Azure and Bing. James can be reached at [email protected].

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