> For the complete documentation index, see [llms.txt](https://nataliekung.gitbook.io/ladder_code/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://nataliekung.gitbook.io/ladder_code/l2er-fen-cha-zhao/first-bad-version.md).

# 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.

分析

二分

往后都是bad，所以每次移动end，然后判断start，再end

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

public class Solution extends VersionControl {
    public int firstBadVersion(int n) {

        int s = 1, e = n;

        while(s + 1 < e){
            int m = s + (e - s)/2;
            if(isBadVersion(m)){
                e = m;
            }else{
                s = m;
            }
        }

        if(isBadVersion(s))
            return s;
        else
            return e;

    }
}
```

```
class Solution:
    def firstBadVersion(self, n):
        """ 
        :type n: int
        :rtype: int
        """
        s,e=0,n
        while s+1<e:
            m = s+(e-s)//2
            if isBadVersion(m):
                e = m
            else:
                s = m
        return s if isBadVersion(s) else e
```

花花木板 左闭右开

```
class Solution:
    def firstBadVersion(self, n):
        """
        :type n: int
        :rtype: int
        """
        s,e = 1,n#因为是开区间，所以应该是N+1，但是怕溢出。
        while s < e:
            m = s + (e-s)//2
            if isBadVersion(m):
                e = m
            else:
                s = m + 1
        return s
```