input: Write a python function to find the longest common substring between two strings.
  assert longest_common_substring('Hawaii', 'Alaska') == 1
  assert longest_common_substring('abcdef', 'abfdef') == 2
  assert longest_common_substring('12345', '67890') == 0
output: def longest_common_substring(str1, str2):
      m, n = len(str1), len(str2)
      longest = 0
      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
                  longest = max(longest, dp[i][j])
              else:
                  dp[i][j] = 0
  
      return longest