Reddit Coding Interview Questions
100 Reddit coding interview problems with full optimal solutions — 34 easy, 52 medium, 14 hard. Every problem ships with multiple approaches (brute-force first, then the optimal), complexity tables for each, company-specific tips on what an Reddit interviewer values, and a FAQ section.
Showing 13 problems of 100
- #1easyfrequently asked
1. Two Sum
Given an array of integers and a target, return indices of the two numbers that add up to target. Reddit uses this as a warm-up to test if you reach for a hash map without prompting — the same reflex that powers vote-deduplication and karma-event lookups.
- #2easyfrequently asked
2. Valid Parentheses
Determine if a string of brackets is balanced. Reddit uses this to test stack intuition — the same data structure that backs their comment-tree depth tracking and markdown parser for self-post bodies.
- #3easyfrequently asked
3. Merge Two Sorted Lists
Merge two sorted linked lists into one sorted list. Reddit uses this to test pointer hygiene — the same skill needed to merge ranked feed streams (hot + new + rising) without losing or duplicating posts.
- #6easyfrequently asked
6. Search Insert Position
Find the index where a target should be inserted in a sorted array. Reddit uses this binary-search variant to gate whether candidates can implement lower_bound correctly — critical for ranked feed insertion when a new post enters Hot.
- #7easyfrequently asked
7. Maximum Subarray
Find the contiguous subarray with the largest sum. Reddit uses this to filter for candidates who recognize Kadane's algorithm — the same running-sum idea that powers their hot-score windowed-vote calculation.
- #9easyfrequently asked
9. Merge Sorted Array
Merge two sorted arrays into the first one in-place. Reddit uses this to test back-pointer technique — the same insight powering their in-place merging of ranked feed segments without allocating a new array per request.
- #10easyfrequently asked
10. Binary Tree Inorder Traversal
Return the inorder traversal of a binary tree. Reddit asks this to test recursive vs. iterative fluency — the same primitive used to flatten their nested comment trees into linear render lists.
- #13easyfrequently asked
13. Maximum Depth of Binary Tree
Compute the depth of a binary tree. Reddit asks this to gauge basic recursion — the same primitive used to measure how deeply a comment tree can nest before they collapse it with 'continue this thread →' links.
- #17easyfrequently asked
17. Best Time to Buy and Sell Stock
Find the max profit from one buy + one sell of a stock-price array. Reddit uses this as a streaming-window template — the same single-pass shape used to find the best upvote burst within a post's lifetime.
- #20easyfrequently asked
20. Linked List Cycle
Detect whether a linked list has a cycle. Reddit uses Floyd's cycle detection (tortoise and hare) to test pointer fluency — the same algorithm used in their comment-graph integrity checker to detect malformed reply cycles.
- #21easyfrequently asked
21. Min Stack
Design a stack that supports push, pop, top, and getMin in O(1). Reddit uses this to test data-structure design — the same auxiliary-state pattern they use to maintain hottest-comment-so-far while streaming an active comment thread.
- #23easyfrequently asked
23. Majority Element
Find the element that appears more than n/2 times in an array. Reddit uses Boyer-Moore voting to test whether candidates recognize specialized algorithms — the same vote-majority pattern matters for their vote-fraud detection (one IP shouldn't dominate a thread).
- #30easyfrequently asked
30. Reverse Linked List
Reverse a singly linked list. Reddit asks this to gauge pointer fluency — the most-asked easy LL problem. The skill maps directly to reversing chronological vote-event chains during fraud investigation.