leetcode_gym/_problems/firstBadVersion.md

layout, title, last_solved, category, leetcode_url, status

layout	title	last_solved	category	leetcode_url	status
page	First Bad Version		binary search	https://leetcode.com/problems/first-bad-version	Attempted

Problem

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. 

Solution

Check for condition at mid and to the right of mid.

    |   |  
1 2 3 4 5
F F F T T

{% highlight python %}

def firstBadVersion(self, n):

low, high = 1, n

while low<=high:
    
    mid = (low+high)//2
    
    if isBadVersion(mid) and not isBadVersion(mid-1):
        return mid
    elif not isBadVersion(mid):
        low = mid+1
    else:
        high = mid-1

return -1

{% endhighlight %}

---

{% highlight python %}

def firstBadVersion(self, n):

left, right = 1, n

while left < right:
    
    mid = (left+right)//2
    
    if isBadVersion(mid):
        
        right = mid
        
    else:
        
        left = mid + 1

return left

{% endhighlight %}