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.

/*

Use a hashmap to gather anagrams, key is sorted string, value is a list that involves the original unsorted string which is same as the key after sorted. 

time:O(Nklogk), space:O(N), N: the number of strings; k: the average length of strings.

*/

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