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) 
    {
        long long begin = 1;
        long long end = n;
        while(begin <= end)
        {
            long long mid = (begin + end)/2;
            if(guess(mid) == -1) end = mid - 1;
            else if(guess(mid) == 1) begin = mid + 1;
            else return mid;
        }
        return -1;
    }
};