374. 猜数字 Guess Number Higher or Lower

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.

题意：已有一个API，可以用来判断一个数与目标数字的大小关系，使用这个API求出目标数字

解法：使用二分法，注意数据类型溢出

int guess(int num);
class Solution {
public:
    int guessNumber(int n) {
        int low = 1;  
        int high = n;  
        int res;  
        int mid;  
        while(high > low){  
            mid = low + (high - low) / 2;  
            res = guess(mid);  
            if(res == 1) low = mid + 1;  
            else if(res == -1) high = mid - 1;  
            else return mid;  
        }  
        return low; 
    }
};

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