input: def longest_common_subsequence(str1: str, str2: str) -> int:
      """
      Find the length of the longest common subsequence between two strings.
  
      >>> longest_common_subsequence("ABACEDF", "ABDCF")
      4
      """
output:     dp = [[0] * (len(str2) + 1) for _ in range(len(str1) + 1)]
      
      for i in range(1, len(str1) + 1):
          for j in range(1, len(str2) + 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])
      
      return dp[-1][-1]