LeetCode 374. 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’ll tell you whether the number is higher or lower.

You call a pre-defined API guess(int num) which returns 3 possible results (-1, 1, or 0):

-1 : My number is lower
 1 : My number is higher
 0 : Congrats! You got it!

Example:

n = 10, I pick 6.

Return 6.

Analysis

二分搜索。要注意使用

//Assume [a,b]
(b - a) / 2 + a

而不是

(a + b) / 2

以防止溢出。

Code

// Forward declaration of guess API.
// @param num, your guess
// @return -1 if my number is lower, 1 if my number is higher, otherwise return 0
int guess(int num);

class Solution {
public:
    int guessNumber(int n) {
        return search(1,n);
    }
private:
    int search(int a, int b){
        int m = (b - a) / 2 + a;
        int g = guess (m);
        if (g == 0) return m;
        else if (g == 1) return search (m + 1, b);
        else if (g == -1) return search (a, m -1);
    }
};

Appendix

• Link: https://leetcode.com/problems/guess-number-higher-or-lower/
• Run Time: 0ms