input: Write a function to find the longest common subsequence of two given sequences, where the sequences are strings. If the given sequences are empty, then return an empty string.
  assert find_lcs("abcdaf", "acbcf")=="abcf"
  assert find_lcs("abcdef", "xyz")==""
  assert find_lcs("aabbcc", "abcabc")=="abc"
  assert find_lcs("", "abc")==""
  assert find_lcs("abc", "")==""
output: def find_lcs(s1, s2):
      if not s1 or not s2:
          return ""
      m, n = len(s1), len(s2)
      dp = [[""] * (n + 1) for _ in range(m + 1)]
      for i in range(1, m + 1):
          for j in range(1, n + 1):
              if s1[i - 1] == s2[j - 1]:
                  dp[i][j] = dp[i - 1][j - 1] + s1[i - 1]
              else:
                  dp[i][j] = max(dp[i - 1][j], dp[i][j - 1], key=len)
      return dp[m][n]