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278. First Bad Version · Space

leetcodeJan 5, 2026

278. First Bad Version

leetcodebinary-searchinteractiveeasy

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v1

01

This is binary search, but on a monotone predicate instead of searching for a target value.

Define a boolean function:

P(v) = isBadVersion(v)

Given the problem statement, P(v) looks like:

False, False, False, True, True, True ...

So we want the first index where P(v) becomes True.

That corresponds to “bisect_left over a boolean array where False < True”:

bisect_left([F,F,F,T,T], True) -> index of first True

v1 code

class Solution:
    def firstBadVersion(self, n: int) -> int:
        left, right = 1, n  # invariant: answer is in [left, right]
        while left < right:
            mid = left + (right - left) // 2
            if isBadVersion(mid):
                # mid is bad, so the first bad is at mid or to the left
                right = mid
            else:
                # mid is good, so the first bad must be to the right
                left = mid + 1
        return left

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