input: def find_longest_common_substring(str1: str, str2: str) -> str:
      """Find the longest common substring in two given strings.
      >>> find_longest_common_substring("Helwallomst", "Welcome")
      'lo'
      """
      max_len = 0
      longest_substr = ""
      len1, len2 = len(str1), len(str2)
      for i in range(len1):
          for j in range(len2):
              length = 0
              while i + length < len1 and j + length < len2 and str1[i + length] == str2[j + length]:
                  length += 1
              if length > max_len:
                  max_len = length
                  longest_substr = str1[i:i + length]
      return longest_substr
output:     max_len = 0
      longest_substr = ""
      len1, len2 = len(str1), len(str2)
      for i in range(len1):
          for j in range(len2):
              length = 0
              while i + length < len1 and j + length < len2 and str1[i + length] == str2[j + length]:
                  length += 1
              if length > max_len:
                  max_len = length
                  longest_substr = str1[i:i + length]
      return longest_substr