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 d 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. 

十分钟尝试

其实是个查询问题，利用折半查找试试，结果正确，我以为折半查找的时候，如果isbadversion为true，需要判断是否前一个为false，才能认定是第一个，其实不用。但是leetcode测试超时，为了避免，可以把left+right／2写成left+(right-left)/2

/* The isBadVersion API is defined in the parent class VersionControl.
      boolean isBadVersion(int version); */

public class Solution extends VersionControl {
    public int firstBadVersion(int n) {
        if(n==1) return n;
        //折半查找
        int low=1;
        int high=n;
        while(low<=high){
            int mid=low+(high-low)/2;
            boolean isBad=isBadVersion(mid);
            if(isBad){
                high=mid-1;
            }
            else{
                low=mid+1;
            }
        
        }
        return low;
    }
}