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

這題使用Binary Search的技巧
/**
* 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
* 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
}