input: Write a function to find the longest common subsequence of two sequences. The function should return the longest common subsequence as a string. The input will be two strings `x` and `y`. If either of the strings is empty, the function should return an empty string.
  assert find_lcs("abcdaf", "acbcf")== "abcf"
  assert find_lcs("abc", "def")== ""
  assert find_lcs("abc", "abc")== "abc"
  assert find_lcs("abc", "")== ""
  assert find_lcs("", "")== ""
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]