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.

分析：给出一系列版本号，若其中一个版本号是bad，那么它后面的所有版本号都是bad。求出第一个bad version。采用二分查找

 public int firstBadVersion(int n) {

       if(n <= 0) return 0;

       int left = 1,right = n,mid;

       while(left <= right){

           mid = left + (right - left) / 2; //防止left + right 太大，溢出，因此要进行变换求解

           if(isBadVersion(mid))

              right = mid - 1;

           else

              left = mid + 1;

       }

       return left;

    }