Guess Number Higher or Lower

Description

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

Code

Time Complexity: $O(\log n)$, Space Complexity: $O(1)$

就是 Binary Search 101。

/** 
 * 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);
 */
 
class Solution {
public:
    int guessNumber(int n) {
        int l = 1, r = n;
        while(l <= r) {
            int m = l + (r - l) / 2;
            if(guess(m) == -1) {
                r = m - 1;
            } else if(guess(m) == 1) {
                l = m + 1;
            } else if(guess(m) == 0) {
                return m;
            }
        }
        return -1;
    }
};

Source

• Guess Number Higher or Lower - LeetCode