LeetCode：374.猜数字大小

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

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