374. Guess Number Higher or Lower

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本文介绍了一个猜数字游戏的实现方案，利用二分搜索算法高效猜测1到n之间的任意数字。每次猜测错误时，游戏会提示目标数字比当前猜测更高还是更低。

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.

This can be easily solved using binary search

int guess(int num);

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