input: Write a function to find the longest common subsequence (LCS) of two given strings. If no common subsequence exists, return an empty string. If both strings are empty, return an empty string.
  assert find_lcs("abcdaf", "acbcf")=="abcf"
  assert find_lcs("abcdef", "ghijkl")==""
  assert find_lcs("a", "a")=="a"
  assert find_lcs("", "")==""
  assert find_lcs("abc", "")==""
output: def find_lcs(s1, s2):
      m, n = len(s1), len(s2)
      dp = [["" for _ in range(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]