LeetCode-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.

public class Solution {
    public List<List<String>> groupAnagrams(String[] strs) {
        if(strs==null || strs.length==0){
            return null;
        }
        List<List<String>> resList=new ArrayList<List<String>>();
        Map<String, List<String>> map=new HashMap<String, List<String>>();
        for(int i=0; i<strs.length; i++){
            String str=strs[i];
            char[] chars=str.toCharArray();
            Arrays.sort(chars);
            String key=new String(chars);
            if(map.containsKey(key)){
                List<String> strList=map.get(key);
                strList.add(str);
                map.put(key,strList);
            }
            else{
                List<String> strList=new ArrayList<String>();
                strList.add(str);
                map.put(key, strList);
            }
        }
        for(String key : map.keySet()){
            resList.add(map.get(key));
        }
        return resList;
    }
}