No.246 質問と回答 on CommonLisp | Codetter（こーどったー）

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;https://yukicoder.me/problems/no/246
(let (
    (numMin 1)
    (numMax 1000000001))
    (loop
        (let ((chkNum (floor(/(+ numMin numMax) 2))))
            (format t "? ~d~%" chkNum)
            (finish-output)
            (if (string=(read-line) "1") (setf numMin chkNum) (setf numMax chkNum))
            (if (= numMin (1- numMax))(progn
                (format t "! ~d~%" numMin)
                (finish-output)
                (return))))))
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