input: Write a python function to find the longest common subsequence between two strings.
  assert lcs("AGGTAB", "GXTXAYB") == "GTAB"
  assert lcs("ABCBDAB", "BDCAB") == "BCAB"
  assert lcs("abcdef", "abc") == "abc"
  assert lcs("XMJYAUZ", "MZJAWXU") == "MJAU"
output: def lcs(X, Y):
      m = len(X)
      n = len(Y)
      
      # Create a 2D array to store lengths of longest common subsequence.
      L = [[0] * (n + 1) for _ in range(m + 1)]
      
      # Build the L[m + 1][n + 1] in bottom-up fashion.
      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])
      
      # Following code is used to print LCS
      index = L[m][n]
      
      # Create a character array to store the lcs string.
      lcs = [""] * (index + 1)
      lcs[index] = ""
      
      # Start from the right-most-bottom-most corner and
      # one by one store characters in lcs[]
      i = m
      j = n
      while i > 0 and j > 0:
          
          # If current character in X[] and Y are same, then
          # current character is part of LCS
          if X[i - 1] == Y[j - 1]:
              lcs[index - 1] = X[i - 1]
              i -= 1
              j -= 1
              index -= 1
          
          # If not same, then find the larger of two and
          # go in the direction of larger value
          elif L[i - 1][j] > L[i][j - 1]:
              i -= 1
          else:
              j -= 1
      
      return "".join(lcs)