input: Write a function to find the longest common subsequence between two strings. The function should take two strings as inputs and return a string representing the longest common subsequence.
  assert longest_common_subsequence("ABCDF", "ABECD")=="ABCD"
  assert longest_common_subsequence("AGGTAB", "GXTXAYB")=="GTAB"
  assert longest_common_subsequence("XMJYAUZ", "MZJAWXU")=="MJAU"
  assert longest_common_subsequence("ABC", "DEF")==""
  assert longest_common_subsequence("", "ABC")==""
output: def longest_common_subsequence(str1, str2):
      m, n = len(str1), len(str2)
      dp = [[0] * (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] + 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 str1[i - 1] == str2[j - 1]:
              lcs.append(str1[i - 1])
              i -= 1
              j -= 1
          elif dp[i - 1][j] > dp[i][j - 1]:
              i -= 1
          else:
              j -= 1
      
      return ''.join(reversed(lcs))