number of all permutations of n elements code example

Example: how to get all permutations of an array

/* to be used something like this:
int [] toBePermuted = new int [] {1, 2, 3, 4};
ArrayList<int[]> a = heap(toBePermuted);
any mention of int [] can be replaced with any other Array of objects */

ArrayList<int []> heap(int [] input) {
  ArrayList<int []> ret = new ArrayList<int []> ();
  ret = generate(input.length, input, ret);
  return ret;
}

ArrayList<int []> generate(int k, int [] a, ArrayList<int []> output) {
  if (k == 1) {
    output.add(a.clone());
  } else {
    output = generate(k-1, a, output);
    for (int i=0; i<k-1; i++) {
      if (k%2 == 0) {
        int temp = a[i];
        a[i] = a[k-1];
        a[k-1] = temp;
      } else {
        int temp = a[0];
        a[0] = a[k-1];
        a[k-1] = temp;
      }
      generate(k-1, a, output);
    }
  }
  return output;
}