input: Write a python function to find the longest common subsequence of two strings.
  assert LongestCommonSubsequence("AGGTAB", "GXTXAYB") == 4
  assert LongestCommonSubsequence("ABC", "AC") == 2
  assert LongestCommonSubsequence("ABCD", "EFGH") == 0
output: def LongestCommonSubsequence(X, Y):
      m = len(X)
      n = len(Y)
      L = [[0] * (n + 1) for _ in range(m + 1)]
  
      for i in range(m + 1):
          for j in range(n + 1):
              if i == 0 or j == 0:
                  L[i][j] = 0
              elif X[i - 1] == Y[j - 1]:
                  L[i][j] = L[i - 1][j - 1] + 1
              else:
                  L[i][j] = max(L[i - 1][j], L[i][j - 1])
  
      return L[m][n]