HP Coding Interview Questions
25 HP coding interview problems with full optimal solutions — 8 easy, 12 medium, 5 hard. Every problem ships with multiple approaches (brute-force first, then the optimal), complexity tables for each, company-specific tips on what an HP interviewer values, and a FAQ section.
- #1easyvery frequently asked
1. Two Sum
HP's firmware and driver teams frequently use look-up tables for O(1) dispatch — Two Sum tests whether you instinctively reach for a hash map instead of a brute-force scan, which matters when your code runs on embedded microcontrollers with tight loop budgets.
- #3mediumfrequently asked
3. Longest Substring Without Repeating Characters
HP's document-processing and OCR pipelines apply sliding-window algorithms for character-sequence analysis in scanned text streams. This problem is a textbook sliding-window exercise — HP uses it to confirm that candidates can translate a brute-force nested loop into an O(n) single-pass scan.
- #4hardoccasionally asked
4. Median of Two Sorted Arrays
HP's quality assurance and performance benchmarking tools compute statistical medians over sorted measurement arrays from multiple test runs or device sources. Achieving O(log(m+n)) time rather than O(m+n) requires binary search across array boundaries — a technique HP senior engineers are expected to command.
- #9easyoccasionally asked
9. Palindrome Number
HP firmware and driver code handles numeric identifiers for device serial numbers, cartridge IDs, and page counters. Palindrome Number tests digit-manipulation skills without string conversion — a meaningful constraint in embedded C where string allocation is expensive.
- #15mediumfrequently asked
15. 3Sum
HP enterprise analytics pipelines perform multi-dimensional range queries and constraint-satisfaction checks on device telemetry. 3Sum is the canonical multi-pointer problem — HP uses it to evaluate whether you can methodically eliminate the cubic brute force down to quadratic through sorting and pointer discipline.
- #20easyfrequently asked
20. Valid Parentheses
Parsing structured expressions is core to HP's firmware configuration parsers and printer command languages (PCL/PJL). Valid Parentheses tests whether you can maintain state with a stack — the same discipline behind device-command validation.
- #21easyfrequently asked
21. Merge Two Sorted Lists
HP's device-driver and print-spooler stacks frequently merge ordered queues of pending jobs from multiple input sources. Merging two sorted linked lists is the fundamental building block — HP uses it to verify that candidates handle pointer manipulation without losing data.
- #23hardoccasionally asked
23. Merge K Sorted Lists
HP's enterprise print-management servers merge sorted job queues from thousands of network printers into a single priority-ordered dispatch stream. Merge K Sorted Lists tests exactly this pattern — combining multiple sorted streams efficiently using a min-heap or divide-and-conquer, skills that matter in HP's large-scale IT infrastructure.
- #42hardoccasionally asked
42. Trapping Rain Water
HP's physical simulation software for environmental testing — used in product durability labs — models fluid accumulation on irregular surfaces. Trapping Rain Water is the algorithmic abstraction of this problem. HP engineers encounter it in the context of dynamic programming, two-pointer optimization, and reasoning about monotonic stack patterns.
- #49mediumfrequently asked
49. Group Anagrams
HP document and content management systems aggregate files with equivalent names or metadata under a unified key. Group Anagrams is a proxy for that canonical key-design question: how do you derive a consistent, collision-free key from unordered data? HP uses it to test hashing intuition.
- #51hardoccasionally asked
51. N-Queens
HP's enterprise scheduling and resource-allocation systems assign non-conflicting resources across dimensions — analogous to placing non-attacking queens on a chessboard. N-Queens is the canonical constraint-satisfaction problem HP uses to test backtracking discipline and pruning intuition in candidates for algorithm-heavy roles.
- #53easyfrequently asked
53. Maximum Subarray
HP uses signal-processing algorithms in imaging and scanning products where finding the strongest contiguous signal window is a real computational task. Maximum Subarray (Kadane's algorithm) is the archetype: HP interviewers use it to test whether you can identify the single-pass greedy insight in a DP-flavored problem.
- #54mediumoccasionally asked
54. Spiral Matrix
HP's scanning and imaging pipelines traverse pixel data in structured patterns — raster scans, zigzag patterns, and spiral reads appear in color-calibration and print-head alignment routines. Spiral Matrix tests whether you can maintain boundaries and direction state without off-by-one errors, a critical skill for HP systems engineers.
- #56mediumfrequently asked
56. Merge Intervals
HP enterprise IT scheduling tools manage maintenance windows, firmware update slots, and print-fleet reservation blocks. Overlapping intervals must be collapsed into non-overlapping ranges before scheduling can proceed. Merge Intervals is the canonical version of this problem, and HP uses it to test sort-then-sweep reasoning.
- #70easyfrequently asked
70. Climbing Stairs
Dynamic programming is central to HP's ink/toner optimization algorithms and cost-scheduling tools for enterprise print fleets. Climbing Stairs is the textbook DP entry point — HP uses it to confirm that candidates understand memoization and state transition before discussing more complex optimization problems.
- #121easyfrequently asked
121. Best Time to Buy and Sell Stock
HP's financial and enterprise analytics teams process time-series data from equipment telemetry and cost-optimization pipelines. This problem tests single-pass reasoning over sequential data — the same pattern used when scanning sensor readings for peak-to-trough performance differences.
- #127hardoccasionally asked
127. Word Ladder
HP's network diagnostics and configuration management tools find shortest transition paths between device states — analogous to Word Ladder's shortest transformation sequence. HP uses this problem to test BFS shortest-path reasoning, graph construction from implicit adjacency, and the ability to prune search space efficiently.
- #139mediumoccasionally asked
139. Word Break
HP's natural-language processing tools for document scanning and search decompose recognized text sequences into valid dictionary words — essentially the Word Break problem. HP uses it to assess dynamic programming depth and the ability to recognize overlapping subproblems in string segmentation.
- #146mediumfrequently asked
146. LRU Cache
HP's firmware and print-spooler stacks manage limited on-device memory with LRU eviction policies for rendered page segments and font caches. LRU Cache is not just a puzzle — it mirrors a real design decision in HP hardware. Interviewers use it to test whether you can compose two data structures to satisfy a compound O(1) requirement.
- #200mediumfrequently asked
200. Number of Islands
HP's imaging and scanning systems detect contiguous regions of ink, toner, or light to distinguish characters and objects on a page. Number of Islands is the archetypal connected-components problem that underlies this region-detection logic — HP uses it to test BFS/DFS grid traversal for systems roles.
- #206easyfrequently asked
206. Reverse Linked List
HP's systems software and firmware teams deal with linked structures constantly — job queues, device-descriptor chains, and print-spooler task lists. Reversing a linked list in-place is a fundamental pointer-manipulation exercise that HP uses to probe candidate comfort with low-level memory operations.
- #207mediumfrequently asked
207. Course Schedule
HP's firmware release pipeline enforces strict build-dependency ordering — drivers must compile before OS modules that depend on them, and cyclic dependencies block production releases. Course Schedule is the canonical cycle-detection-in-a-directed-graph problem that mirrors this real constraint. HP uses it to evaluate topological-sort and DFS fluency.
- #238mediumfrequently asked
238. Product of Array Except Self
HP's numerical calibration and imaging-pipeline software computes normalization factors that exclude each sensor's own reading. Product of Array Except Self — without using division — is a direct test of prefix/suffix product reasoning that appears naturally in HP's signal-processing and correction workflows.
- #322mediumfrequently asked
322. Coin Change
HP enterprise subscription and billing systems compute optimal denomination splits when processing payments or credits. Coin Change is the canonical unbounded-knapsack DP problem that HP uses to evaluate whether candidates can set up a state table, define a clean recurrence, and reason about the base case.
- #347mediumfrequently asked
347. Top K Frequent Elements
HP support and diagnostics tools rank error codes and telemetry events by frequency to surface the most impactful hardware failures. Top K Frequent Elements tests whether you can combine frequency counting with an efficient selection mechanism — a pattern used across HP's observability stack.