49. Group Anagrams

Given an array of strings, group anagrams together.

For example, given: ["eat", "tea", "tan", "ate", "nat", "bat"], 
Return:

[
  ["ate", "eat","tea"],
  ["nat","tan"],
  ["bat"]
]

Note: All inputs will be in lower-case.

题目含义：将给定的一组字符串数组，按照同构词（相同字母组成的单词）分类，每组单词按照字典排序。

思路：将每个字符串升序排列，这样同构字符串排列后的顺序一致，可以作为map的key使用

    public List<List<String>> groupAnagrams(String[] strs) {
         Map<String,List<String>> stringMap = new HashMap<>();
        List<List<String>> result = new ArrayList<>();
        if (strs.length == 0) return result;

        for (String str:strs)
        {
            char[]chars = str.toCharArray();
            Arrays.sort(chars);
            String newString = new String(chars);
            List<String> strings = stringMap.getOrDefault(newString,new ArrayList<>());
            strings.add(str);
            stringMap.put(newString,strings);
        }
        return new ArrayList<>(stringMap.values());       
    }