input: 
  Write a function to find the longest common subsequence between two strings. If the strings are empty, return 0. 
  def lcs(str1, str2):
      # Your code here
      pass
  
  assert lcs("ABACEDF", "ABDCF") == 4
  assert lcs("ABC", "AC") == 2
  assert lcs("", "ABC") == 0
  assert lcs("ABC", "") == 0
  assert lcs("DE", "FGHI") == 0
  
output:     
      if not str1 or not str2:
          return 0
      
      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])
      
      return dp[m][n]
  