49. 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.

Solution:

If two strings s1 and s2 are anagram, sort(s1) will be equal to sort(s2). So we can use sorted element as a key to the HashMap.

We go through the input array. For each s, we sort it and check if the HashMap has this key. If so, s is an anagrams and we put it to corresponding value list.



Code:

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