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.
[code lang="java"]
class Solution {
    public List<List<String>> groupAnagrams(String[] strs) {
        
    }
}
[code]

Idea – 1

Sorted version of a group of anagrams is unique, so we shall use a hashmap to group the anagrams where key will be the sorted version. Since all strings consist of lowercase letters only, we use counting sort to perform sorting in O(m) time – m is number of chars in the longest string. Time complexity is then O(n\cdot m) and space complexity is also O(n\cdot m). 
[code lang="java"]
class Solution {
    public List<List<String>> groupAnagrams(String[] strs) {
        List<List<String>> ans = new ArrayList<>();
        if(strs==null || strs.length==0)
        {
            return ans;
        }
        
        HashMap<String, List<String>> map = new HashMap<>();
        for(String w : strs)
        {
            // 26 unique chars
            char[] counts = new char[26];
            for(int i = 0; i < w.length(); ++i)
            {
                ++counts[ w.charAt(i)-'a' ];
            }
            
            String sorted = new String(counts);
            List<String> group = map.getOrDefault(sorted, new ArrayList<>());
            group.add(w);
            
            map.put(sorted, group);
        }
        
        for(String key : map.keySet())
        {
            ans.add(map.get(key));
        }
        
        return ans;
    }
}
[code]

Runtime: 9 ms, faster than 89.34% of Java online submissions for Group Anagrams.Memory Usage: 40.7 MB, less than 93.53% of Java online submissions for Group Anagrams.

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