String of Unique Characters

Counting characters​

Often you will need to count the frequency of characters in a string. The most common way of doing that is by using a hash table/map in your language of choice. 

If you need to keep a counter of characters, a common mistake is to say that the space complexity required for the counter is O(n). The space required for a counter of a string of latin characters is O(1) not O(n). This is because the upper bound is the range of characters, which is usually a fixed constant of 26. The input set is just lowercase Latin characters.