input: Write a function to find the longest common subsequence of two sequences. If no common subsequence exists, return an empty string.
  assert find_lcs("abcdaf", "acbcf")=="abcf"
  assert find_lcs("AGGTAB", "GXTXAYB")=="GTAB"
  assert find_lcs("hello", "world")==""
  assert find_lcs("", "test")==""
  assert find_lcs("abc", "abc")=="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]