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
}