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Just Helped a Student Ace HRT OA! Practical Strategies for 4 Questions

Last week, I walked a student through Hudson River Trading (HRT)’s online assessment (OA), and one key takeaway stood out: HRT doesn’t test flashy algorithms.

Instead, it focuses on logical rigor and edge case handling — with 4 questions in 90 minutes, nailing the details already wins you half the battle.

Below is a complete walkthrough of the interview process and step-by-step strategies for each question — designed for students preparing for quant or tech role OAs.

No fluff, just actionable insights.

I. HRT Interview Process: OA Is the Critical Filter

HRT’s hiring moves fast, typically with three stages — and passing the OA basically gets you one foot in the door for technical interviews:

1. Online Assessment (OA)

  • The most crucial stage.
  • 4 questions in 90 minutes, covering:
    • Algorithms
    • Logic
    • Data structures
    • Simulation
  • Goal: Solve at least 3 questions for a strong chance at the next round.
  • Emphasis: Write robust, efficient, and readable code under time pressure.

2. Technical Interviews

Usually 2–3 rounds, combining coding, math/logic, and discussion:

  • Coding: Clean implementation & clear reasoning. Topics: arrays, strings, probability simulation.
  • Math/Logic: Focuses on discrete reasoning & accuracy. E.g., randomness problems, time complexity estimation.
  • Discussion: Often references OA solutions — e.g., “What if the input size scales up?”

3. Final Round / Culture Fit

A hybrid Trader + Engineer style conversation covering:

  • System efficiency
  • Risk control
  • Collaboration
  • Quick logical reasoning

II. Detailed Strategies for the 4 OA Questions

Q1: Find Peaks

Problem:

Given an integer array, find all indices of "peaks."

Rules:

  • Middle elements: greater than both neighbors.
  • First element: greater than second.
  • Last element: greater than second-to-last.

Core Strategy:

  • Traverse once and handle 3 cases:
    • arr[0] > arr[1]
    • arr[n-1] > arr[n-2]
    • arr[i] > arr[i-1] && arr[i] > arr[i+1]
  • Record all valid indices.

Key Focus:

  • Don’t overlook first/last elements.
  • Time complexity: O(n), no extra space.

Q2: Count Substrings Divisible by 3

Problem:

Given a numeric string (e.g., "123"), count all substrings that:

  1. Are divisible by 3
  2. Have no leading zeros

Core Strategy:
Use prefix sum modulo 3 instead of brute force.

Steps:

  1. Compute prefix sums mod 3.
  2. If prefix[i] == prefix[j], substring s[j..i-1] is divisible by 3.
  3. Use a hash map to count occurrences of each remainder.
  4. Handle single '0' specially (valid), but skip "01", "003", etc.

Key Focus:

  • Prefix sum modulo technique (common OA trick)
  • Handle leading zeros carefully

Q3: Grid Simulation

Problem:

Simulate grid movement from a starting cell.

Grid contains:

  • Obstacles (-1)
  • Teleport points
  • Goal cell

Return:

  • -1 → hit obstacle
  • -2 → loop detected
  • Step count → reached goal

Core Strategy:
Simulation + loop detection.

Steps:

  1. Store obstacles & teleports in hash sets for O(1) lookups.
  2. Move step by step, tracking current position and visited cells.
  3. On each move, check in order:
    • Out of bounds?
    • Obstacle? → return -1
    • Visited? → return -2
    • Goal? → return step count
    • Teleport? → jump and continue

Key Focus:

  • Maintain visited set correctly.
  • Order of checks matters (avoid false loop detection).

Q4: Largest Square in Histogram

Problem:

Given an array of bar heights, find the area of the largest square that fits inside the histogram.

Core Strategy:
Modify the monotonic stack method for rectangles to handle squares.

Steps:

  1. Use a monotonic increasing stack to find the first smaller bar to the left and right.
  2. Compute width: right[i] - left[i] - 1
  3. Square side length = min(height[i], width)
  4. Area = side * side

Key Focus:

  • Classic monotonic stack pattern.
  • Use min(height, width) for square logic.

III. The Key to Passing HRT OA

Many students know the material but fail due to:

  • Time pressure
  • Missing small details (e.g., forgetting to handle leading zeros or not marking visited cells)

If that’s you, consider Programhelp’s Seamless Voice-Assisted Support — a practical way to avoid those “I knew this but froze” moments.

Why It Works

  • Comprehensive Coverage: Assisted OAs for HRT, Jane Street, Citadel, IMC, Optiver, Two Sigma, etc.
  • Smooth Experience: Real-time, lag-free voice guidance (e.g., “Check teleport before counting steps” or “Update prefix sum hash map here.”)
  • Proven Results: Students consistently AC 3+ questions in 90 minutes, securing interview invitations.

📩 Get the latest HRT OA question bank (including recent recall questions)

& our “90-Minute Time Management Guide” to sharpen your pacing and accuracy.

IV. Quick Preparation Tips

  • Prioritize pacing: 4 questions / 90 minutes → ~20 minutes each. Skip and return if stuck.
  • Plan edge cases first: Arrays, strings, grids — HRT loves edge conditions.
  • Master classic patterns: Prefix sums, stacks, loop detection — these recur with small twists.

💬 Need help with OA implementation or prep planning?

Leave a comment or send a message — we’ll help you build confidence, strategy, and speed before your next assessment.

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