0278 First Bad Version

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Solved at: 230128

Question

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.

Solution

swift

class Solution : VersionControl {    func firstBadVersion(_ n: Int) -> Int {        var high : Int = n        var low  : Int = 0        while(low < high - 1) {            let midpoint = (high + low) / 2            if isBadVersion(midpoint) {                high = midpoint            } else {                low = midpoint            }        }        if isBadVersion(low) {            return low        }        return high    }}

class Solution : VersionControl {    func firstBadVersion(_ n: Int) -> Int {        var high : Int = n        var low  : Int = 0        while(low < high - 1) {            let midpoint = (high + low) / 2            if isBadVersion(midpoint) {                high = midpoint            } else {                low = midpoint            }        }        if isBadVersion(low) {            return low        }        return high    }}

Results

Runtime 0 ms, Beats 100%
Memory 13.8 MB, Beats 32.28%

Complexity Analysis

Runtime: $log_2 n$ given isBadVersion is $O(1)$ Space Complexity: $O(n)$

Takeaways

When using binary search, we must consider the case where low < high but when low = high - 1 and thus never exiting the loop.