First Bad Version

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 returns whether version is bad. Implement a function to find the first bad version. You should minimize the number of calls to the API. (Question is taken from LeetCode)

Approach: Its like finding an element in a sorted array. We can use binary search here.

Implementation in C#:

        public int FirstBadVersion(int n)

        {

            int start = 1, end = n;

            while (start <= end)

            {

                int mid = start + (end - start) / 2; // To avoid overflow

                if (IsBadVersion(mid) && (mid == 1 || !IsBadVersion(mid - 1)))

                {

                    return mid;

                }

                else if (IsBadVersion(mid))

                {

                    end = mid - 1;

                }

                else

                {

                    start = mid + 1;

                }

            }

            return -1;

        }

Complexity: O(logn)