[leetcode] 278. First Bad Version 解题报告

题目链接：https://leetcode.com/problems/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.

思路：一个简单的二分查找，一直奇怪为啥超时，看了一下discus才发现原来是（left + right）/2会引起整型溢出，需要用mid = left + (right - left)/2;

代码如下：

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

class Solution {
public:
    int firstBadVersion(int n) {
        int left = 1, right = n, mid;
        while(left <= right)
        {
            mid = left + (right - left)/2;
            if(isBadVersion(mid) == false)//不是坏的版本
                left = mid +1;
            else
                right = mid-1;
        }
        return left;
    }
};