49. Group Anagrams

Problem:

Given an array of strings, group anagrams together.

Example:

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

Note:

• All inputs will be in lowercase.
• The order of your output does not matter.

Analysis:

What's in common of anagrams is they are the same if we sort them. So use a map, key is the sorted string, value is the anagram list. 

Solution:

class Solution {
    public List<List<String>> groupAnagrams(String[] strs) {
        List<List<String>> res = new ArrayList<>();
        if (strs == null || strs.length == 0)
            return res;
        Map<String, List<String>> map = new HashMap<>();
        
        for (int i = 0; i < strs.length; i++) {
            char[] chars = strs[i].toCharArray();
            Arrays.sort(chars);
            String sortedString = new String(chars);
            map.computeIfAbsent(new String(chars), k -> new ArrayList<>()).add(strs[i]);
        }
        
        for (Map.Entry<String, List<String>> entry: map.entrySet()) {
            res.add(entry.getValue());
        }
        return res;
    }
    
    
}