[Leetcode 278]第一个出错的版本 First Bad Version

 

【题目】

一个版本出错，后续版本皆错，找到第一个出错的版本

You are a product manager and currently leading a team to develop a new product. Unfortunately, the latest version of your product fails the quality check. Since each version is developed based on the previous version, all the versions after a bad version are also bad.

Suppose you have n versions [1, 2, ..., n] and you want to find out the first bad one, which causes all the following ones to be bad.

You are given an API bool isBadVersion(version) which returns whether version is bad. Implement a function to find the first bad version. You should minimize the number of calls to the API.

 

Example 1:

Input: n = 5, bad = 4
Output: 4
Explanation:
call isBadVersion(3) -> false
call isBadVersion(5) -> true
call isBadVersion(4) -> true
Then 4 is the first bad version.

Example 2:

Input: n = 1, bad = 1
Output: 1

 

【思路】

二分

终于有感觉能自己很快写出来一次AC了…………虽然是easy也纪念下…………

 

【代码】

 

// The API isBadVersion is defined for you.
// bool isBadVersion(int version);

class Solution {
public:
    int firstBadVersion(int n) {
        //注意，true的时候是bad version
        int left=0;int right=n;
        while(left<right){
            int mid=left+(right-left)/2;
            if(isBadVersion(mid))
                right=mid;
            else
                left=mid+1;
        }
        return right;
    }
};