input: Write a python function to find the longest common subsequence (LCS) of two strings.
  assert find_lcs("abcdaf", "acbcf") == "abcf"
  assert find_lcs("AGGTAB", "GXTXAYB") == "GTAB"
  assert find_lcs("ABC", "AC") == "AC"
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]