input: Write a python function to find the longest common substring between two strings.
  assert longestSubstring("abcdef", "abfdef") == 2
  assert longestSubstring("hello", "yellow") == 4
  assert longestSubstring("abcdxyz", "xyzabcd") == 4
  assert longestSubstring("abc", "def") == 0
output: def longestSubstring(str1, str2):
      m, n = len(str1), len(str2)
      dp = [[0] * (n + 1) for _ in range(m + 1)]
      max_length = 0
  
      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
                  max_length = max(max_length, dp[i][j])
  
      return max_length