## Questions

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 three possible results:

`-1`

: Your guess is higher than the number I picked (i.e.`num > pick`

).`1`

: Your guess is lower than the number I picked (i.e.`num < pick`

).`0`

: your guess is equal to the number I picked (i.e.`num == pick`

).

Return *the number that I picked*.

## Algorithm

The trivial solution is you try each number each time and check if it's the one you want. However, this will waste the `guess()`

.

Guess function not only return if the number is what we want, but also can tell us it's higher and lower. And we are give a numbers from 1 to n which is **sorted**. Comparing, sorted numbers, these may lead you to the binary search.

Since we don't have a target to compare, but we do have `guess()`

who can direct us, so we still can decide which side to go.

## Code

/** * Forward declaration of guess API. * @param num your guess * @return -1 if num is higher than the picked number * 1 if num is lower than the picked number * otherwise return 0 * int guess(int num); */ public class Solution extends GuessGame { public int guessNumber(int n) { int left = 1; int right = n; while (left <= right) { int mid = left + (right - left) / 2; if (guess(mid) == 0) { return mid; } else if (guess(mid) == 1) { left = mid + 1; } else { right = mid - 1; } } return left - 1; } }