Given an array of strings, group anagrams together.

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

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