原题链接：Leecode 374. 猜数字大小

/** 
 * Forward declaration of guess API.
 * @param  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
 * int guess(int num);
 */

class Solution {
public:
    int guessNumber(int n) {
        int l=1,r=n;
        while(l<r)
        {
            int mid=l+(r-l)/2;
            if(guess(mid)==0)  return mid;
            else if(guess(mid)==1)  l=mid+1;
            else r=mid-1;
        }   
        return l;
    }
};