input: Write a function to find the longest common subsequence (LCS) of two strings. If the inputs are `x` and `y`, the function should return the LCS as a string. If either of the strings is empty, the function should return an empty string.
  assert find_lcs("abcdaf", "acbcf")=="abcf"
  assert find_lcs("abcdef", "ghijkl")==""
  assert find_lcs("abc", "abc")=="abc"
  assert find_lcs("abc", "def")==""
  assert find_lcs("", "abc")==""
output: def find_lcs(x, y):
      if not x or not y:
          return ""
      m, n = len(x), len(y)
      dp = [[""] * (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] + x[i - 1]
              else:
                  dp[i][j] = max(dp[i - 1][j], dp[i][j - 1], key=len)
      return dp[m][n]