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 11 problems of 100
- #87hardsometimes asked
87. Word Ladder
Find the shortest transformation sequence from beginWord to endWord (one-letter change per step). Reddit uses this BFS problem to test graph exploration — the same shortest-path shape used when tracing comment-edit chains across moderator interventions.
- #88hardrarely asked
88. Median of Two Sorted Arrays
Find the median of two sorted arrays in O(log(min(m, n))). Reddit uses this to test binary-search-over-partitions — relevant when merging sorted vote-tally arrays from two shards to find a percentile cutoff.
- #90hardrarely asked
90. Regular Expression Matching
Implement regex matching for '.' and '*'. Reddit uses this DP problem to test state-machine thinking — the same shape used when implementing custom rule-based content filters in their AutoModerator engine.
- #91hardsometimes asked
91. Reverse Nodes in k-Group
Reverse nodes of a linked list in k-groups. Reddit uses this to test linked-list manipulation under partial-reverse rules — relevant when re-ordering feed chunks in their ranked-page generation.
- #92hardrarely asked
92. Longest Valid Parentheses
Find the length of the longest valid parentheses substring. Reddit uses this stack-based DP problem to test sophisticated index tracking — the same shape used when validating longest valid markdown nesting in user input.
- #93hardrarely asked
93. Sudoku Solver
Solve a 9x9 Sudoku puzzle in-place using backtracking. Reddit uses this to test constraint-propagation backtracking — the same shape used when assigning thousands of users to a finite set of moderator-approval slots under multiple non-conflict constraints.
- #94hardrarely asked
94. First Missing Positive
Find the smallest missing positive integer in O(n) time and O(1) space. Reddit uses this for the cyclic-sort trick — clever in-place manipulation relevant to compact range-checks during shard-key allocation.
- #96hardrarely asked
96. Wildcard Matching
Match a string against a pattern with '?' and '*'. Reddit uses this DP problem to test pattern-matching state — the same shape used in their AutoModerator regex-lite engine for subreddit-specific rules.
- #97hardrarely asked
97. N-Queens
Place n queens on an n×n board so none attack each other. Reddit uses this backtracking classic to test constraint-encoded DFS — the same shape they use when scheduling non-conflicting moderator-action sequences across overlapping subreddits.
- #98hardsometimes asked
98. Largest Rectangle in Histogram
Find the largest rectangle in a histogram. Reddit uses this monotonic-stack classic to test sophisticated stack technique — the same shape used in their time-series anomaly detection on vote-burst histograms.
- #99hardrarely asked
99. Maximal Rectangle
Find the largest all-1s rectangle in a binary matrix. Reddit uses this to test reduction from 2D to 1D histogram — the same shape used when computing the largest contiguous active region on their heatmap of subreddit activity over time.