leetcode 374. Guess Number Higher or Lower

解题思路：
类似二分查找，注意是谁比谁小，目标值比猜的值小，返回-1
原题目：

We are playing the Guess Game. The game is as follows:I pick a number from 1 to n. You have to guess which number I picked.Every time you guess wrong, I'll tell you whether the number is higher or lower.You call a pre-defined API guess(int num) which returns 3 possible results (-1, 1, or 0):-1 : My number is lower 1 : My number is higher 0 : Congrats! You got it!Example:n = 10, I pick 6.Return 6.

AC解，C++代码，菜鸟一个，请大家多多指正

// Forward declaration of guess API.// @param num, your guess// @return -1 if my number is lower, 1 if my number is higher, otherwise return 0int guess(int num);class Solution {public:    int guessNumber(int n) {        int upperbound = n;        int lowerbound = 1;        while (lowerbound <= upperbound) {            int middle = lowerbound + ((upperbound -lowerbound) >> 1);            int ret = guess(middle);            if (ret < 0) {                upperbound = middle - 1;            }            else if (ret > 0) {                lowerbound = middle + 1;            }            else {                return middle;            }        }    return 0;   // can't find    }};