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Count White Balls After K Steps

Last updated: Apr 27, 2026

Quick Overview

This question evaluates understanding of modular arithmetic, parity tracking, periodic behavior on circular data structures, and the design of efficient preprocessing for repeated queries.

  • hard
  • Voleon
  • Coding & Algorithms
  • Software Engineer

Count White Balls After K Steps

Company: Voleon

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: hard

Interview Round: Technical Screen

You are given a ring of `N` positions indexed from `0` to `N-1`. - At each position `i`, there is initially one ball whose color is either white or black. - Each position may also contain a flip gate, represented by `gate[i]` where `1` means a gate is present and `0` means no gate. - In one synchronous step, every ball moves one position clockwise. - Whenever a ball crosses a position containing a flip gate, its color toggles. For each query, you are given an integer `K` and must determine how many balls are white after exactly `K` steps. Additional details: - A ball starting at index `i` will be located at `(i + K) mod N` after `K` steps. - Its final color depends only on whether it crosses an even or odd number of gates during those `K` moves. - `K` may be much larger than `N`, so simulating step by step is too slow. Design an algorithm that preprocesses the ring and answers each query efficiently, ideally in `O(1)` time per query after preprocessing.

Quick Answer: This question evaluates understanding of modular arithmetic, parity tracking, periodic behavior on circular data structures, and the design of efficient preprocessing for repeated queries.

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Voleon
Feb 10, 2026, 12:00 AM
Software Engineer
Technical Screen
Coding & Algorithms
11
0

You are given a ring of N positions indexed from 0 to N-1.

  • At each position i , there is initially one ball whose color is either white or black.
  • Each position may also contain a flip gate, represented by gate[i] where 1 means a gate is present and 0 means no gate.
  • In one synchronous step, every ball moves one position clockwise.
  • Whenever a ball crosses a position containing a flip gate, its color toggles.

For each query, you are given an integer K and must determine how many balls are white after exactly K steps.

Additional details:

  • A ball starting at index i will be located at (i + K) mod N after K steps.
  • Its final color depends only on whether it crosses an even or odd number of gates during those K moves.
  • K may be much larger than N , so simulating step by step is too slow.

Design an algorithm that preprocesses the ring and answers each query efficiently, ideally in O(1) time per query after preprocessing.

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