[LeetCode] 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.

典型的二分查找问题。

// 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) {
        int lo = 1, hi = n;
        while (lo <= hi) {
            int mid = lo + (hi - lo) / 2;
            int res = guess(mid);
            if (res == 0)
                return mid;
            else if (res == -1)
                hi = mid - 1;
            else if (res == 1)
                lo = mid + 1;
        }
        return 0;
    }
};
// 0 ms