# 374. Guess Number Higher or Lower

Easy
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.
Example 1:
Input: n = 10, pick = 6
Output:
6
Example 2:
Input: n = 1, pick = 1
Output:
1
Example 3:
Input: n = 2, pick = 1
Output:
1
Constraints:
• `1 <= n <= 231 - 1`
• `1 <= pick <= n`

### 解題

/**
* Forward declaration of guess API.
* @return -1 if num is higher than the picked number
* 1 if num is lower than the picked number
* otherwise return 0
* func guess(num int) int;
*/
func guessNumber(n int) int {
start := 1
end := n
for start <= end {
mid := (start + end) / 2
switch guess(mid) {
case 0:
return mid
case 1:
start = mid+1
default:
end = mid-1
}
}
return -1
}