https://leetcode.com/problems/guess-number-higher-or-lower/

Solved: 7m00s

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.

Solution

1/** 
2 * Forward declaration of guess API.
3 * @param  num   your guess
4 * @return 	     -1 if num is lower than the guess number
5 *			      1 if num is higher than the guess number
6 *               otherwise return 0
7 * int guess(int num);
8 */
9
10class Solution {
11public:
12    int bsearch(int b, int e) {
13        while(b <= e) {
14            int mid = b + (e-b)/2;
15            if(guess(mid) == 0) return mid;
16            if(guess(mid) == -1) e = mid-1;
17            if(guess(mid) == 1) b = mid+1;
18        }
19        return -1;
20    }
21    int guessNumber(int n) {
22        return bsearch(1, n);
23    }
24};

Time Complexity

• Binary search havles every time - logN
• O(logN)

Space Complexity

• No extra storage was used.
• O(1)