> For the complete documentation index, see [llms.txt](https://jimmylin1991.gitbook.io/practice-of-algorithm-problems/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jimmylin1991.gitbook.io/practice-of-algorithm-problems/binary-search/374.-guess-number-higher-or-lower.md).

# 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
```

```cpp
// 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;
}
```