input: Write a python function to find the longest common subsequence of two strings.
  assert longest_common_subsequence("ABCDF", "ABECD") == "ABD"
  assert longest_common_subsequence("AGGTAB", "GXTXAYB") == "GTAB"
  assert longest_common_subsequence("abc", "abc") == "abc"
  assert longest_common_subsequence("abc", "def") == ""
output: def longest_common_subsequence(s1, s2):
      m, n = len(s1), len(s2)
      dp = [[0] * (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] + 1
              else:
                  dp[i][j] = max(dp[i - 1][j], dp[i][j - 1])
  
      lcs = []
      i, j = m, n
      while i > 0 and j > 0:
          if s1[i - 1] == s2[j - 1]:
              lcs.append(s1[i - 1])
              i -= 1
              j -= 1
          elif dp[i - 1][j] > dp[i][j - 1]:
              i -= 1
          else:
              j -= 1
  
      return ''.join(reversed(lcs))