LeetCode374. Guess Number Higher or Lower——二分查找

一、题目

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

二、题解

/** * 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 mid = l + ((r - l) >> 1);if(guess(mid) == 0) return mid;else if(guess(mid) == 1) l = mid + 1;else r = mid - 1;}return 0;}
};