input: Write a function to find the longest common subsequence between two strings.
  assert LongestCommonSequence("AGGTAB", "GXTXAYB") == "GTAB"
  assert LongestCommonSequence("ABCDGH", "AEDFHR") == "ADH"
  assert LongestCommonSequence("ABC", "AC") == "AC"
output: def LongestCommonSequence(X, Y):
      m = len(X)
      n = len(Y)
      
      # Create a table to store lengths of longest common subsequence
      L = [[None]*(n+1) for i in range(m+1)]
      
      # Build 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 construct the 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)