input: Write a function to find the longest common subsequence between two strings. The function should return the longest subsequence as a string. If the given strings are empty, the function should return an empty string.
  assert longest_common_subsequence("ABCDF", "ABECD")=="ABC"
  assert longest_common_subsequence("XYZ", "XYZ")=="XYZ"
  assert longest_common_subsequence("ABC", "DEF")==""
  assert longest_common_subsequence("", "ABC")==""
  assert longest_common_subsequence("XYZ", "")==""
output: def longest_common_subsequence(str1, str2):
      if not str1 or not str2:
          return ""
      m, n = len(str1), len(str2)
      dp = [[""] * (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] = max(dp[i - 1][j], dp[i][j - 1], key=len)
      return dp[m][n]