Registration - Algorithm

Here's your approach:

$1.$ Read $n$, the number of registration requests.

$2.$ Create an empty map from string to integer.

$3.$ For each of the $n$ requests, read the username.

$4.$ Check if the username exists in the map.

$5.$ If not found, output OK and insert username with value 0.

$6.$ If found, get its count, output username concatenated with count, then increment the count in the map. This solution processes each request in $O(\log n)$ time for map operations, giving you $O(n \log n)$ total time. Maps provide fast lookup and update, making the solution both clean and fast.