49. Group Anagrams.java

Given an array of strings, group anagrams together.
For example, given: ["eat", "tea", "tan", "ate", "nat", "bat"], 
Return:
[
  ["ate", "eat","tea"],
  ["nat","tan"],
  ["bat"]
]
思路:
1.map<String, List<String>>，key是排过序的String, List是相同的anagram
2. 比较是否anagram，只需要把每个String转成charArray再排序

class Solution {
    public List<List<String>> groupAnagrams(String[] strs) {
        List<List<String>> res = new ArrayList<>();
        if(strs == null || strs.length == 0){
            return res;
        }
        HashMap<String, List<String>> map = new HashMap<>();
        for(String str : strs){
            char[] chars = str.toCharArray();
            Arrays.sort(chars);
            String s = new String(chars);
            if(!map.containsKey(s)){
                map.put(s, new ArrayList<String>());
            }
            map.get(s).add(str);
        }
        res.addAll(map.values());
        return res;
    }