You should aim for a solution with O(m * n) time and O(m) space, where m is the number of strings and n is the length of the longest string.

A naive solution would be to sort each string and group them using a hash map. This would be an O(m * nlogn) solution. Though this solution is acceptable, can you think of a better way without sorting the strings?

By the definition of an anagram, we only care about the frequency of each character in a string. How is this helpful in solving the problem?

We can simply use an array of size O(26), since the character set is a through z (26 continuous characters), to count the frequency of each character in a string. Then, we can use this array as the key in the hash map to group the strings.