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