Time limit per test: 1 second Memory limit per test: 256 megabytes Goal ----- There is a hidden integer x with 1 ≤ x ≤ n that you must determine. What you can do ---------------- 1) Ask membership questions (up to 53 total): - Choose any non-empty set S ⊆ {1, 2, …, n}. - Ask whether x ∈ S. - The judge replies “YES” if x ∈ S, otherwise “NO”. 2) Make guesses (up to 2 total): - Output a single number as your guess for x. - The reply is always truthful: • “:)” if your guess equals x (you must terminate immediately). • “:(” if your guess is wrong. Noisy answers & guarantee -------------------------- - Not all “YES”/“NO” answers are guaranteed to be truthful. - However, for every pair of consecutive questions, at least one answer is correct. - This remains true across guesses: if you ask a question, then make a guess, then ask another question, the “consecutive questions” rule applies to those two questions surrounding the guess. - Guesses themselves are always judged correctly. Adaptive x ---------- - The judge does not fix x in advance; it may change over time. - Changes are constrained so that all previous responses remain valid and consistent with: • the rule “for each two consecutive questions, at least one answer is correct,” and • the correctness of guess judgments. Input ----- - A single integer n (1 ≤ n ≤ 100000), the maximum possible value of x. Interactive protocol (I/O format) ---------------------------------- To ask a question about a set S: - Print a line: “? k s1 s2 … sk” • k = |S| (k ≥ 1) • s1, s2, …, sk are distinct integers in [1, n] - Flush output immediately. - Read a single word reply: “YES” or “NO”. To make a guess for x: - Print a line: “! g” where g is your guess (1 ≤ g ≤ n). - Flush output immediately. - Read the judge’s reply: • “:)” if correct — your program must terminate immediately. • “:(” if incorrect — you may continue if you still have remaining queries/guesses. Flushing -------- After every printed line, flush the output to avoid idleness/timeout: - C++: fflush(stdout) or cout.flush() - Java: System.out.flush() - Pascal: flush(output) - Python: sys.stdout.flush() - See language docs for others. Limits ------ - Maximum questions: 53 - Maximum guesses: 2 - n up to 100000 Important notes ---------------- - Because at least one of every two consecutive question answers is correct, you can design strategies that compare adjacent answers to filter lies. - Guesses are always reliable; use them sparingly (you only have 2). - Note that this problem has a scoring system. You are graded based on the # of queries you use. The lower the # of queries you use, the higher the score you get. Example -------- Input 6 (Sequence of interactions as seen by the contestant) ? 5 1 2 5 4 3 NO ! 6 :( ? 4 1 2 3 4 NO ! 5 :) Explanation - If the first question’s “NO” had been truthful, x would have to be 6. - The guess “! 6” receives “:(“, so 6 is not the answer. Therefore, the first answer must have been a lie. - By the guarantee, the next question’s answer must then be truthful. - From “? 4 1 2 3 4” with reply “NO”, we conclude x ∉ {1,2,3,4}; combined with “6 is wrong”, x must be 5. - The final guess “! 5” is confirmed with “:)”.