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.

这道题是一道典型的猜价格的问题，根据对方说高了还是低了来缩小范围，最简单快速的方法就是折半搜索法，原理很简单，参见代码如下：

// Forward declaration of guess API.
// @param num, your guess
// @return -1 if my number is lower, 1 if my number is higher, otherwise return 0
int guess(int num);

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

类似题目：

Guess Number Higher or Lower II 

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