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 20 problems of 100
- #31mediumfrequently asked
31. Add Two Numbers
Add two big integers represented as linked lists (least-significant-digit first). Reddit asks this to test linked-list arithmetic with carry — the same kind of pointer-walking they use to merge cross-shard counter chunks.
- #32mediumfrequently asked
32. Longest Substring Without Repeating Characters
Find the longest substring without repeating characters. Reddit asks this to test sliding-window technique — the same pattern used to find the longest run of non-duplicate IPs hitting an endpoint (a fraud-detection primitive).
- #34mediumfrequently asked
34. Container With Most Water
Given heights, find the two lines that form the container holding the most water. Reddit uses this two-pointer problem to test greedy/monotone intuition — analogous to choosing the two endpoints of a time-window to maximize comment volume.
- #35mediumfrequently asked
35. 3Sum
Find all unique triplets that sum to zero. Reddit uses this to test sort + two-pointer + dedup — the same triple-key correlation used in their abuse-detection to find triple-coincidence patterns (IP + user-agent + timing).
- #37mediumfrequently asked
37. Remove Nth Node From End of List
Remove the Nth-from-end node in a single pass. Reddit asks this to test the two-pointer-with-gap technique — the same windowed-walk used in their rate-limiter to expire the Nth-most-recent event.
- #41mediumfrequently asked
41. Search in Rotated Sorted Array
Search for a target in a rotated sorted array in O(log n). Reddit asks this to test binary-search variants — the same skill used when their pagination cursor wraps around a sorted vote-timestamp index.
- #49mediumfrequently asked
49. Group Anagrams
Group strings that are anagrams of each other. Reddit uses this to test hash-key design — the same shape used when clustering near-duplicate post titles for spam detection (sort the letters to get a normalized fingerprint).
- #52mediumfrequently asked
52. Merge Intervals
Merge overlapping intervals. Reddit uses this canonical interval problem to test sort + scan — the same shape used when merging consecutive vote-window aggregations into a single hot-score period.
- #60mediumfrequently asked
60. Subsets
Generate all 2^n subsets of a set of distinct integers. Reddit uses this to test backtracking and bitmask enumeration — the same enumeration used when computing all flag-combination buckets for feature-rollout cohorts.
- #64mediumfrequently asked
64. Binary Tree Level Order Traversal
Return level-order traversal of a binary tree as a list of lists. Reddit uses this BFS warm-up to test queue technique — the same shape used in their comment-tree renderer which paginates by depth level.
- #67mediumfrequently asked
67. Validate Binary Search Tree
Determine whether a binary tree is a valid BST. Reddit uses this to test the bounded-recursion pattern — the same shape used when validating that a comment-tree's parent timestamps satisfy a strict-monotonic constraint.
- #68mediumfrequently asked
68. Word Break
Determine if a string can be segmented into words from a dictionary. Reddit uses this DP problem to test the prefix-suffix decomposition pattern — the same shape used when tokenizing usernames into recognized subreddit references during mention detection.
- #69mediumfrequently asked
69. LRU Cache
Design an LRU cache with O(1) get and put. Reddit uses this to test data-structure composition (hash + doubly-linked-list) — the same primitive used in their hot-post cache eviction layer in front of Postgres.
- #70mediumfrequently asked
70. Number of Islands
Count the number of connected components of 1s in a grid. Reddit uses this DFS/BFS classic to test grid traversal — the same shape used when clustering coordinated abuse cells in their fraud-detection IP-grid.
- #71mediumfrequently asked
71. Course Schedule
Determine if you can finish all courses given prerequisite pairs (cycle detection in a DAG). Reddit uses this to test topological-sort intuition — the same shape used when validating cross-shard data-migration dependency graphs.
- #73mediumfrequently asked
73. Implement Trie (Prefix Tree)
Implement a trie supporting insert, search, and startsWith. Reddit uses this to test prefix-tree design — the same data structure they use in their autocomplete service for subreddit and username completion.
- #74mediumfrequently asked
74. Kth Largest Element in an Array
Find the kth-largest element. Reddit uses this to test heap and quickselect — the same primitive that powers their top-k post selector for ranking feed pages.
- #79mediumfrequently asked
79. Lowest Common Ancestor of a Binary Tree
Find the lowest common ancestor of two nodes in a binary tree. Reddit uses this canonical LCA problem to test post-order recursion — the same shape they use to find the nearest common parent comment when merging conflicting moderator threads.
- #80mediumfrequently asked
80. Product of Array Except Self
Return an array where each element is the product of all others (no division). Reddit uses this to test prefix/suffix product technique — the same shape used in their cross-shard counter reconciliation where you compute totals excluding one shard.
- #81mediumfrequently asked
81. Top K Frequent Elements
Return the k most frequent elements in an array. Reddit uses this to test count + heap design — the exact shape of their trending-words detector in subreddit titles over a time window.