Guess Number Higher or Lower Problem & Solution

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 will tell you whether the number I picked is higher or lower than your guess. You call a pre-defined API int guess(int num), which returns 3 possible results:

• -1: The number I picked is lower than your guess (i.e. pick < num).
• 1: The number I picked is higher than your guess (i.e. pick > num).
• 0: The number I picked is equal to your guess (i.e. pick == num).

Return the number that I picked.

C++ Solution

The runtime complexity of the algorithm is $O(\log{}n)$.

#pragma GCC optimize("Ofast")
#pragma GCC optimization("unroll-loops")

static const int _=[](){std::ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);return 0;}();

/**
* Forward declaration of guess API.
* @return 	     -1 if num is lower than the guess number
*			         1 if num is higher than the guess number
*               otherwise return 0
* int guess(int num);
*/
class Solution {
public:
int guessNumber(int n) {
int low = 1;
int high = n;
while (low < high) {
int mid = low + (high - low) / 2;
int result = guess(mid);

if (result == 0) {
return mid;
} else if (result > 0) {
low = mid + 1;
} else {
high = mid;
}
}

return low;
}
};