CODE

public class Solution {
  public bool CheckInclusion(string s1, string s2) {

    /* only check window size that is length of s1 */
    int window_size = s1.Length;
    int max_window_size = s1.Length + 1;

    int i = 0;
    int j = i + window_size;
    bool permutationFound = false;

    while (window_size < max_window_size && !permutationFound)
    {
      while (j <= s2.Length && !permutationFound)
      {
        permutationFound = checkPermutation(s1, s2, i, j-1);
        i++;
        j++;
      }

      window_size++;
      i = 0;
      j = i + window_size;
    }
    return permutationFound;
  }

  bool checkPermutation(string s1, string s2, int s, int e)
  {
    char[] s2Window = new char[e - s + 1];

    char[] s1Chars = new char[s1.Length];

    for (int i = 0; i < s2Window.Length; i++)
    {
      s2Window[i] = s2[s++];
      s1Chars[i] = s1[i];
    }
    Array.Sort(s2Window);
    Array.Sort(s1Chars);

    for (int i = 0; i < s2Window.Length; i++)
      if (s2Window[i] != s1Chars[i])
        return false;

    return true;
  }
}