Here's your approach:
Read the username string.
Create an empty set to store characters.
Loop through each character in the username.
Insert each character into the set.
After the loop, check the size of the set.
If the size is odd, output IGNORE HIM.
If the size is even, output CHAT WITH HER. This solution runs in time where is the length of the username. Each character is processed once. Set insertion and size checking are fast operations.