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Hudson River Trading·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
Apr 2026Remote

Summary

Got a simulation-style coding problem from HRT that looks deceptively clean on the surface but has a lot of moving parts to track simultaneously. Not sure how I did.

Questions Asked (1)

Q1

You have a 1D array of length L. Watchers and players both move at speed 1 per unit time. Watchers start facing left and reverse direction at given timestamps (unsorted). A player is frozen whenever a watcher shares their position. A player wins if they reach position L by time T. Given initial positions of all watchers and players, plus the reversal timestamps and end time, return the count of players who successfully reach the end.

Algorithms & Data StructuresSystem Design
Author's notes

The reversal timestamps being unsorted was the first thing that tripped me up.

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AI HintsAI Generated

Suggested Approach

First, clarify the problem constraints and assumptions, then propose an efficient simulation or event-based approach. Discuss how to model watcher movement with direction changes and detect collisions with players, and finally compute the count of successful players.

Pro tip: Mention that sorting the reversal timestamps and using a sweep-line or priority queue can handle unsorted events efficiently, and that early termination or pruning can optimize the simulation.

1. Clarify Requirements and Constraints

Ask about the range of L, T, number of watchers/players, and whether positions are integers. Confirm that watchers and players move simultaneously and that freezing is instantaneous.

2. Model Watcher Movement

Represent each watcher's state (position, direction) and process reversal timestamps in sorted order. Use a function to compute a watcher's position at any time t.

3. Detect Player-Watcher Collisions

For each player, determine if they are frozen at any time before reaching L. This can be done by checking if the player's path intersects any watcher's path, considering direction changes.

4. Simulate or Compute Efficiently

Use event-based simulation or mathematical intersection to avoid O(T) time. For each player, compute the earliest time they would be frozen; if that time is before they reach L, they fail.

5. Count and Return

Iterate over all players, determine if they succeed, and return the count. Discuss time complexity and potential optimizations.

Key Points to Mention

  • Sorting reversal timestamps to handle unsorted input
  • Using a priority queue or sweep-line for event processing
  • Modeling watcher movement as piecewise linear functions
  • Checking for collisions by solving equations of motion
  • Handling simultaneous events and edge cases (e.g., player and watcher at same position at t=0)
  • Time complexity analysis and optimization strategies

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.