Leetcode 278. First Bad Version

Leetcode 278. First Bad Version

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.

Example:Given n = 5, and version = 4 is the first bad version.

call isBadVersion(3) -> false
call isBadVersion(5) -> true
call isBadVersion(4) -> true

Then 4 is the first bad version.

题目大意：
给定n个产品前若干个为好，后若干个为次，使用API找到其中的第一个次品，尽可能减少API的调用次数。

解题思路：
数组有序，查找问题，典型的二分。时间复杂度O(logn).

代码：

bool isBadVersion(int version); //API

class Solution {
public:
    int firstBadVersion(int n) {
        int l = 1, r = n;
        while(l <= r)
        {
            int mid = (r - l) / 2 + l;
            if(isBadVersion(mid) == true)
                r = mid - 1;
            else if(isBadVersion(mid) == false)
                l = mid + 1;
        }
        return isBadVersion(l) == true ? l : r;
    }
};