private static boolean permutation(
      final String s1,
      final String s2) {
    if (s1.length() != s2.length()) {
      return false;
    }
    int[] letters = new int[256];// ASCII
    for (char ch : s1.toCharArray()) {
      letters[ch]++;
    }

    for (int i = 0; i < s2.length(); i++) {
      if (--letters[s2.charAt(i)] < 0) {
        return false;
      }
    }

    return true;
  }