input: Write a python function to find the length of the longest common subsequence (LCS) of two given strings.
  assert lcs("abcdaf", "zbcdf") == 4
  assert lcs("AGGTAB", "GXTXAYB") == 4
  assert lcs("ABC", "AC") == 2
output: def lcs(str1, str2):
      m = len(str1)
      n = 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]