49. Group Anagrams

Given an array of strings, group anagrams together.

Example:

Input: ["eat", "tea", "tan", "ate", "nat", "bat"],
Output:
[
  ["ate","eat","tea"],
  ["nat","tan"],
  ["bat"]
]

Note:

• All inputs will be in lowercase.
• The order of your output does not matter.

判断anagram词：排序后一样。

时间：O(N)，空间：O(N)

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, Integer> map = new HashMap<>();
        for(String str : strs) {
            char[] ch = str.toCharArray();
            Arrays.sort(ch);
            String s = new String(ch);
            if(map.containsKey(s)) {
                List<String> list = res.get(map.get(s));
                list.add(str);
            } else {
                List<String> list = new ArrayList<>();
                list.add(str);
                map.put(s, res.size());
                res.add(list);
            }
        }
        return res;
    }
}