【一天一道LeetCode】#374. Guess Number Higher or Lower

一天一道LeetCode

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（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’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.

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

（二）解题

题目大意：猜数游戏，1~n里面的数，首先你心里想一个数num，让对方猜，对方每猜一个数你都要判断比num大还是小，直到对方猜对！

解题思路：采用二分搜索法来猜数效率最高。注意int guess(int num)函数用来返回大还是小。

// 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) {
        int i = 0;
        int j = n;
        //二分查找法
        while(i<=j)
        {
            int mid = i+(j-i)/2;//取中点
            int flag = guess(mid);//返回大于、小于或等于
            if(flag == 0) return mid;
            else if(flag == -1) j = mid-1;
            else i = mid+1;
        }
        return 0;
    }
};