278. First Bad Version

问题描述：

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

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

Then 4 is the first bad version.

解题思路：

可以用二分法来查找，但是当我计算mid的时候

若使用mid = （left + right)/2 就会超时。

据说是当left和right都很大的时候会出现这种情况

为了避免这种情况

使用mid = left + (right - left)/2来计算

代码：

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

class Solution {
public:
    int firstBadVersion(int n) {
        int left = 1;
        int right = n;
        while(left < right){
            int mid = left + (right - left) / 2;
            if(isBadVersion(mid)){
                right = mid;
            }else{
                left = mid+1;
            }
        }
        return left;
    }
};