You're given a username as a string. Your task: count the number of distinct characters in it. If the count is odd, output IGNORE HIM. If even, output CHAT WITH HER. Simple counting, but I'll show you the right tool.
This problem teaches you when sets beat arrays. You could loop through the string and manually check each character, but sets give you automatic deduplication. Add all characters to a set, then check its size.
Before reading on, think: if the username is "abbac", how many distinct characters are there? How would you count them by hand? That mental process is what sets automate.