Hudson River Trading OA 2026 Full Breakdown | CodeSignal 4 Problems + Solutions Guide

Introduction

Just wrapped up the latest Hudson River Trading (HRT) OA with a student targeting 2026 Quant / Trading roles. This round reflects a very typical HRT-style assessment: clean problem design, strong emphasis on data structures, and heavy focus on implementation accuracy.

The student completed all 4 problems in 25 minutes with full AC, leaving sufficient time for edge-case validation — which is often the key differentiator in these OAs.

OA Overview

Platform: CodeSignal
Duration: 70 minutes
Total Questions: 4
Difficulty: Balanced (Q1–Q2 easier, Q3–Q4 more involved)

There is also another HRT OA variant: 150 minutes with 3 problems, where the last question includes follow-ups and requires deeper reasoning.

Problem 1: Obstacle Placement & Interval Checking

You are given two types of operations:

[1, x]: Place an obstacle at coordinate x
[2, x, size]: Check whether an interval centered at x with length size can be placed without overlapping obstacles

The interval extends symmetrically: [x - (size - 1), x + (size - 1)].

Core idea: Use an ordered set to maintain obstacle positions. For each query, locate the closest obstacles around x using binary search.

Time complexity: O(log n) per operation.

Problem 2: Earliest Day Reaching Target Visits

Given an array visits representing daily traffic, return the earliest index where cumulative visits ≥ target. If never reached, return -1.

Core idea: Simple prefix sum with one pass.

Edge cases:

target ≤ 0 → return 0
empty array → return -1

This is essentially a warm-up problem — must be solved quickly.

Problem 3: Lexicographically Smallest String via Prefix/Suffix Reversals

You are allowed to reverse any prefix or suffix of the string any number of times. Find the lexicographically smallest string achievable.

Core idea: Enumerate all possible prefix and suffix reversals and track the smallest result.

Time complexity: O(n²), which is acceptable under constraints.

Key insight: Although operations seem unlimited, the reachable states are still bounded enough for brute force.

Problem 4: Bubble Elimination Game Simulation

This is the hardest problem in the set.

Game rules:

Click a cell → eliminate that cell and all diagonally adjacent cells with the same color (4 diagonal directions only)
After elimination, apply gravity per column (cells fall downward, empty spaces filled with 0)

Core idea: