input: Write a function to find the longest common subsequence between two strings. The function should return the length of the longest common subsequence.
  
  assert lcs("ABACEDF", "ABDCF") == 4
  assert lcs("AGGTAB", "GXTXAYB") == 4
  assert lcs("ABC", "DEF") == 0
  assert lcs("ABC", "ABC") == 3
  assert lcs("", "ABC") == 0
output: def lcs(s1, s2):
      m, n = len(s1), len(s2)
      dp = [[0] * (n + 1) for _ in range(m + 1)]
      
      for i in range(1, m + 1):
          for j in range(1, n + 1):
              if s1[i - 1] == s2[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]