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;
    }
}

二刷：注意能把containsKey用在object上的条件

class Solution {
    public List<List<String>> groupAnagrams(String[] strs) {
        List<List<String>> resList = new ArrayList<List<String>>();
        if(strs == null || strs.length == 0){
            return resList;
        }
        Map<Character, Integer> charMap = new HashMap<>();
        Map<Map<Character, Integer>, List<String>> strMap = new HashMap<>();

        for(String str : strs){
            Map<Character, Integer> key = generateCharMap(str);
            //The reason we can use containsKey here is because the key implements equals function
            if(strMap.containsKey(key)){
                List<String> list = strMap.get(key);
                list.add(str);
                strMap.put(key, list);
            }
            else{
                List<String> list = new ArrayList<>();
                list.add (str);
                strMap.put(key, list);
            }
        }
        for(Map<Character, Integer> key : strMap.keySet()){
            resList.add(strMap.get(key));
        }
        return resList;
    }

    private Map<Character, Integer> generateCharMap(String str){
        Map<Character, Integer> map = new HashMap<>();
        for(char c : str.toCharArray()){
            map.put(c, map.getOrDefault(c,0)+1);
        }
        return map;
    }

}