374. Guess Number Higher or Lower

Problem

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 <= 2^31 - 1

• 1 <= pick <= n

Solution

/** 
 * 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) {
        return helper(1,n);
    }

    public int helper(int start, int end){
        if(start == end) return start;
        int mid = Math.toIntExact(((long)start+(long)end)/2);
        int r = 0;
        if(guess(mid) == 0) r = mid;
        else if(guess(mid) == 1) r = helper(mid+1, end);
        else if(guess(mid) == -1) r = helper(start, mid-1);
        return r;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).