LeetCode（278）First Bad Version

题目

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 will return whether version is bad. Implement a function to find the first bad version. You should minimize the number of calls to the API.

Credits:

Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

分析

题目要求在产品的n个版本中找到第一个bad version。在所有版本中，若是当前i版本是bad，则后续i+1版本也是bad。

考察二分查找的应用。

AC代码

// Forward declaration of isBadVersion API.
bool isBadVersion(int version);

class Solution {
public:
    int firstBadVersion(int n) {
        if (n < 1)
            return -1;
        int lhs = 1, rhs = n;

        while (lhs < rhs)
        {
            int mid = lhs + (rhs - lhs) / 2;
            //找到bad version
            if (isBadVersion(mid))
            {
                rhs = mid;
            }
            //mid并不是bad version 则first bad肯定在右边
            else{
                lhs = mid + 1;
            }
        }//while
        return lhs;
    }
};