input: Write a python function to find the length of the longest common subsequence between two strings.
  assert LCS("ABCBDAB", "BDCAB") == 4
  assert LCS("AGGTAB", "GXTXAYB") == 4
  assert LCS("AAAA", "AA") == 2
output: def LCS(X, Y):
      m = len(X)
      n = len(Y)
      dp = [[0] * (n + 1) for _ in range(m + 1)]
  
      for i in range(1, m + 1):
          for j in range(1, n + 1):
              if X[i - 1] == Y[j - 1]:
                  dp[i][j] = dp[i - 1][j - 1] + 1
              else:
                  dp[i][j] = max(dp[i - 1][j], dp[i][j - 1])
  
      return dp[m][n]