LeetCode 3842: Toggle Light Bulbs

Meta Data

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Difficulty: easy First Attempt: 2026-02-16

• Submissions: Wrong Answer → Accepted

Approach

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Each appearance toggles once, so the bulbs left on are those appearing an odd number of times. Simulate with a set: remove if present, add if not (Hash Table Template 1). The answer must be ascending.

My Attempts

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Attempt 1 returned list(light), but a set has no order. For [8, 1] the answer is [1, 8]; it returned [8, 1]. Adding sorted passed.

Solution

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class Solution:
    def toggleLightBulbs(self, bulbs: List[int]) -> List[int]:
        on = set()
        for b in bulbs:
            on ^= {b}                      # remove if present, add if not
        return sorted(on)

Complexity

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Time O(n + k log k) with k bulbs on at the end, space O(k).

Mistakes to Avoid

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• Converting a set to a list does not guarantee order; sort when the problem asks.

Interview Explanation

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Every appearance toggles, so bulbs appearing an odd number of times stay on. I toggle with a symmetric difference on a set and return it sorted. Time O(n log n).