input: Write a python function to find the longest common subsequence between two strings.
  assert LongestCommonSubsequence("ACDFSCD", "ACBCD", len("ACDFSCD"), len("ACBCD")) == 4
  assert LongestCommonSubsequence("ABC", "AC", len("ABC"), len("AC")) == 2
  assert LongestCommonSubsequence("ABC", "DEF", len("ABC"), len("DEF")) == 0
output: def LongestCommonSubsequence(X, Y, m, n):
      if m == 0 or n == 0:
          return 0
      elif X[m-1] == Y[n-1]:
          return 1 + LongestCommonSubsequence(X, Y, m-1, n-1)
      else:
          return max(LongestCommonSubsequence(X, Y, m, n-1), LongestCommonSubsequence(X, Y, m-1, n))