input: Write a function to find the longest common substring in two input strings. Return the longest common substring as a string. If there is no common substring, return an empty string.
  
  def longest_common_substring(str1, str2):
      longest_length = 0
      longest_substring = ""
      for i in range(len(str1)):
          for j in range(len(str2)):
              length = 0
              while (i + length < len(str1) and j + length < len(str2) and str1[i + length] == str2[j + length]):
                  length += 1
              if length > longest_length:
                  longest_substring = str1[i:i + length]
                  longest_length = length
      return longest_substring
  
  assert longest_common_substring("", "") == ""
  assert longest_common_substring("abc", "def") == ""
  assert longest_common_substring("abcdef", "abc") == "abc"
  assert longest_common_substring("Helwallomst", "Welcome") == "ell"
  assert longest_common_substring("banana", "anana") == "anana"
output: def longest_common_substring(str1, str2):
      longest_length = 0
      longest_substring = ""
      for i in range(len(str1)):
          for j in range(len(str2)):
              length = 0
              while (i + length < len(str1) and j + length < len(str2) and str1[i + length] == str2[j + length]):
                  length += 1
              if length > longest_length:
                  longest_substring = str1[i:i + length]
                  longest_length = length
      return longest_substring