input: Write a python function to find the longest common substring among a list of strings.
  assert find_longest_common_substring(["flower","flow","flight"]) == "fl"
  assert find_longest_common_substring(["dog","racecar","car"]) == ""
  assert find_longest_common_substring(["interview","interrupt","integrate","internal"]) == "inte"
output: def find_longest_common_substring(strs):
      if not strs:
          return ""
      
      def longest_common_prefix(str1, str2):
          min_length = min(len(str1), len(str2))
          for i in range(min_length):
              if str1[i] != str2[i]:
                  return str1[:i]
          return str1[:min_length]
      
      common_substr = strs[0]
      for i in range(1, len(strs)):
          common_substr = longest_common_prefix(common_substr, strs[i])
          if not common_substr:
              break
      
      return common_substr