同分异构词归类

Given an array of strings, group anagrams together.

Example:

Input: ["eat", "tea", "tan", "ate", "nat", "bat"],
Output:
[
  ["ate","eat","tea"],
  ["nat","tan"],
  ["bat"]
]
用一个hash表来存储字典序同分词，后面用一个容器接原词；
例如 ate eat tea ，字典序都是aet，后面的second为一个vector容器，承接 ate eat tea
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