CODE

public class Solution {
  public int CharacterReplacement(string s, int k) {
    char[] sArray = new char[s.Length];
    char[] sArraySorted = new char[s.Length];
    for (int i = 0; i < s.Length; i++)
    {
      sArray[i] = s[i];
      sArraySorted[i] = s[i];
    }

    Array.Sort(sArraySorted);

    int count = 1;

    char[] letters = new char[s.Length];
    int[] freqs = new int[s.Length];

    int freq = 1;
    int ind = 0;


    for (int i = 1; i < sArraySorted.Length; i++)
    {
      Console.Write(sArraySorted[i-1]);
      if (sArraySorted[i] == sArraySorted[i-1])
        count++;
      else
      {
        letters[ind] = sArraySorted[i-1];
        freqs[ind++] = count;
        count = 1;
      }
      if (i == sArraySorted.Length - 1)
      {
        letters[ind] = sArraySorted[i];
        freqs[ind] = count;
      }
    }

    Array.Sort(freqs, letters);
    Array.Reverse(freqs);
    Array.Reverse(letters);
    int l = 0;

    int max = freqs[0];
    int c = 0;

    /* sort chars by frequency, get top 6 most frequent */
    for (int i = 0; i < freqs.Length; i++)
    {
      if (c < 7 && i < (freqs.Length - 1) && freqs[i] > max - 8)
        c++;
      else
        break;
    }

    char[] lettersToTest = new char[c];
    for (int i = 0; i < c; i++)
      lettersToTest[i] = letters[i];

    int total_length = 0;
    int max_window_size = s.Length;

    /* go through sliding windows for all 6 most frequent letters */
    for (int m = 0; m < lettersToTest.Length; m++)
    {
      int window_size = 1;

      while (window_size < max_window_size)
      {
        int i = 0;            
        int j = i + window_size;

        int repl = k;
        int start = 0;

        while (j < s.Length)
        {
          int length = 0;
          repl = k;

          while (i <= j)
          {
            /* if letter not in frequent letters or is but not currently being 
            checked, can replace it if sufficient k */
            if ((!Array.Exists(lettersToTest, element => element == s[i]) || 
              (Array.Exists(lettersToTest, element => element == s[i]) && 
              s[i] != lettersToTest[m])) && repl > 0)
            {
              repl--;
              length++;
              i++;
            }
            else if (s[i] == lettersToTest[m])
            {
              i++;
              length++;
            }
            else
            {                       
              if (length > total_length)
                total_length = length;
              length = 0;
              i++;
            }
          }

          if (length > total_length)
            total_length = length;

          i = start++;
          j = i + window_size;
        }
        window_size++;
      }
    }
    return total_length;
  }
}