input: def longest_common_substring(str1: str, str2: str) -> str:
      """
      Find the longest common substring in two given strings.
      >>> longest_common_substring("Helwallomst", "Welcome")
      'ell'
      """
output:     max_len = 0
      max_substr = ""
      for i in range(len(str1)):
          for j in range(len(str2)):
              k = 0
              while i + k < len(str1) and j + k < len(str2) and str1[i + k] == str2[j + k]:
                  k += 1
              if k > max_len:
                  max_len = k
                  max_substr = str1[i:i + k]
      return max_substr