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function registration_system(requests)
username_count := empty map from string to integer
for each username in requests
if username not in username_count then
output "OK"
username_count[username] := 0
else
count := username_count[username]
output username + count
username_count[username] := count + 1
return
The map tracks state across all requests. Each username remembers how many times it appeared. Notice the pattern: check existence, then either insert or update.