249. Group-Shifted-Strings.md

Description

Given a string, we can "shift" each of its letter to its successive letter, for example: "abc" -> "bcd". We can keep "shifting" which forms the sequence:

"abc" -> "bcd" -> ... -> "xyz"

Given a list of strings which contains only lowercase alphabets, group all strings that belong to the same shifting sequence.

Example:

Input: ["abc", "bcd", "acef", "xyz", "az", "ba", "a", "z"],
Output: 
[
  ["abc","bcd","xyz"],
  ["az","ba"],
  ["acef"],
  ["a","z"]
]

---

我们要有一个统一的标准，将每个单词都转化成以z开头的单词，这样才能判断两个单词是否是同一组。

• 写一个函数convertToZstart实现上面的功能。

• 用一个defaultdict来实现分组保存的功能，其中键为z开头的单词，值为该组的所有单词（一个列表）。

  python solution

  from collections import defaultdict
  
  
  class Solution:
      def groupStrings(self, strings):
          """
          :type strings: List[str]
          :rtype: List[List[str]]
          """
  
          # a的ascii码是97 z的ascii码是122
          def convertToZstart(word):
              step = ord('z') - ord(word[0]) # step为第一个字母转化成z需要的步数。
              return "".join(map(lambda c: chr(ord(c) + step if ord(c) + step < 123 else ord(c) + step - 26), word))  # 如果当前字母加上step后大于ord(z)，需要减去26。
  
          # 用一个defaultdict来存储z开头的字符串。
          dd = defaultdict(list)
          for word in strings:
              dd[convertToZstart(word)].append(word)
          return [value for value in dd.values()]