374. 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 :

Input: n = 10, pick = 6
Output: 6

// 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);

// Binary search
int guessNumber(int n) { // time: O(logn); space: O(1)
    int low = 1, high = n;
    while (low <= high) {
        int mid = low + (high - low) / 2, res = guess(mid);
        if (res == 0) return mid;
        else if (res == 1) low = mid + 1;
        else high = mid - 1;
    }
    return -1;
}