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0278 First Bad Version

Solved at: 2023-01-28

First Bad Version - LeetCode


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.


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


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

Complexity Analysis

Runtime: log2nlog_2 n given isBadVersion is O(1)O(1) Space Complexity: O(n)O(n)


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

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