## Description

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, 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.
```

## Explanation

Use binary search to find first bad version with O(logn) time complexity.

## Java Solution

```
/* 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 start = 1;
int end = n;
while (start + 1 < end) {
int mid = start + (end - start) / 2;
if (isBadVersion(mid)) {
end = mid;
} else {
start = mid;
}
}
if (isBadVersion(start)) {
return start;
}
return end;
}
}
```

## Python Solution

```
# The isBadVersion API is already defined for you.
# @param version, an integer
# @return a bool
# def isBadVersion(version):
class Solution:
def firstBadVersion(self, n):
"""
:type n: int
:rtype: int
"""
start = 1
end = n
while start + 1 < end:
mid = start + (end - start) // 2
if isBadVersion(mid) == True:
end = mid
else:
start = mid
if isBadVersion(start):
return start
else:
return end
```

- Time complexity: O(log
*n*). The search space is halved each time, so the time complexity is O(log*n*). - Space complexity: O(1)

You never defined isBadVersion() method, My system is showing “the name “isBadVerstion” does not exists. how do I resolve this, I am writing in c#