input: Write a python function to find the longest common subsequence of two strings.
  assert longest_common_subsequence("abcde", "ace") == "ace"
  assert longest_common_subsequence("abc", "abc") == "abc"
  assert longest_common_subsequence("abc", "def") == ""
  assert longest_common_subsequence("abcbdab", "bdcaba") == "bdab"
output: def longest_common_subsequence(str1, str2):
      m, n = len(str1), len(str2)
      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 str1[i-1] == str2[j-1]:
                  dp[i][j] = dp[i-1][j-1] + str1[i-1]
              else:
                  dp[i][j] = dp[i-1][j] if len(dp[i-1][j]) > len(dp[i][j-1]) else dp[i][j-1]
  
      return dp[m][n]