374. 猜数字大小

/** 
 * Forward declaration of guess API.
 * @param {number} num   your guess
 * @return                 -1 if num is lower than the guess number
 *                         1 if num is higher than the guess number
 *                       otherwise return 0
 * var guess = function(num) {}
 */

/**
 * @param {number} n
 * @return {number}
 */
var guessNumber = function(n) {
    let left = 1, right=n;
    while(left<right){
        let mid = Math.floor((left+right)/2);
        let check = guess(mid);
        if(check === 0){
            return mid;
        }else if(check === -1){
            right=mid;
        }else{
            left=mid+1;
        }
    }
    return left;
};