Bulb switcher

Math Brainteaser

55.5% acceptance

Jan 12, 2026

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3230

There are n bulbs that are initially off. You first turn on all the bulbs, then you turn off every second bulb. On the third round, you toggle every third bulb (turning on if it's off or turning off if it's on). For the ith round, you toggle every i bulb. For the nth round, you only toggle the last bulb. Return the number of bulbs that are on after n rounds.

Solution

Rust

Time O(1)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn bulb_switch(n: i32) -> i32 {
    (n as f64).sqrt() as i32
  }
}