Problems

Pair Sum in a Sorted Array

Algorithms & Data Structures · The two-pointer warm-up every loop still asks.

Merge Overlapping Intervals

Algorithms & Data Structures · Sort, then sweep — the pattern behind a dozen calendar problems.

Design an LRU Cache

Algorithms & Data Structures · Hash map plus doubly linked list, all operations O(1).

Course Schedule

Algorithms & Data Structures · Cycle detection dressed up as a scheduling question.

Max Width

Algorithms & Data Structures · Greedy line packing and fiddly space math — full text justification.

Round Numeric String Values

Algorithms & Data Structures · Arbitrary-precision rounding — floats need not apply.

Violation Log Analyzer

Algorithms & Data Structures · Streaming design: recent counts, top-k, and sliding-window bans.

Nested Set Equality

Algorithms & Data Structures · Order and duplicates vanish at every depth — canonicalize.

Assign Pins to Shortest Columns

Algorithms & Data Structures · The masonry-layout greedy: shortest column wins, leftmost on ties.

Collect Pins from Reachable Boards

Algorithms & Data Structures · BFS over the bipartite pin-board graph without building board-to-board edges.

Stream Line Reader

Algorithms & Data Structures · Reassemble lines from arbitrary chunks, then settle the balances they carry.

Escape-Room Leaderboard

Algorithms & Data Structures · Best-time standings with live ranks — the data-structure trade-off talk.

Rebalance Experiment Buckets with Minimal Reassignment

Algorithms & Data Structures · Meet new group sizes while disrupting as few buckets as possible.

List Unallocated Experiment Buckets

Algorithms & Data Structures · Interval sweep for the free ranges — independent of bucket-space size.

Design a Smallest-Free ID Allocator

Algorithms & Data Structures · Smallest-free allocation under a capacity that moves at runtime.

Cross-Reference Logs to Flag Spam Numbers

Algorithms & Data Structures · A hash join across two logs — and saying that word out loud.

Sparse Matrix Storage, Addition, and Multiplication

Algorithms & Data Structures · Storage proportional to nonzeros — and the cancellation trap.

Select the Nearest Eligible Elevator

Algorithms & Data Structures · A requirements-gathering problem wearing a simulation costume.

Decide Target via Subsequence Plus/Multiply Expression

Algorithms & Data Structures · Expression search with precedence — the (total, pending product) trick.

Compute Reachable Cells for a Cleaning Robot

Algorithms & Data Structures · Slide-until-blocked physics: rest positions are the states, not cells.

Maximize Boxes Stored Through One Entrance

Algorithms & Data Structures · Prefix-min ceilings plus an exchange-argument greedy.

Mark and Compact a Heap-Indexed Subtree

Algorithms & Data Structures · Garbage collection in miniature: mark a subtree, compact, remap.

Implement Single-Tab Browser History Navigation

Algorithms & Data Structures · A cursor over one visit stack: truncate forward history, clamp at both ends.

Design Adjustable ID Allocator

Algorithms & Data Structures · Pack named ID ranges, then resize with all-or-nothing shifts.

Settle Debts

Algorithms & Data Structures · Net the balances, then match debtors to creditors greedily.

Reconstruct Itinerary

Algorithms & Data Structures · Hierholzer's walk, plus the loop question that fails candidates.

Access-Log Query System

Algorithms & Data Structures · Two sorted indexes and a binary search you write by hand.

Bus Routes: Minimum Transfers

Algorithms & Data Structures · BFS over routes, not stops — clear each stop as you expand it.

Board Reachability with Exact Jumps

Algorithms & Data Structures · Two out-edges per index; BFS answers both parts in one pass.

Restaurant Seating: Free Intervals

Algorithms & Data Structures · A +ppl/−ppl sweep; occupancy is constant between events.

Count Pins on a Two-Column Screen

Algorithms & Data Structures · Only n scroll offsets matter — slide the window to a pin's top.

Count Subarrays with Score Below K

Algorithms & Data Structures · Positives make sum × length monotone — a two-pointer window.

Count Objects in a Pixel Grid via an API

Algorithms & Data Structures · Flood fill through an opaque API — isSameObject is the catch.

Roads with Switches

Algorithms & Data Structures · Open roads cost 0, closed cost 1 — textbook 0-1 BFS.

Bank Tellers: Wait Time

Algorithms & Data Structures · A min-heap simulation, then binary search on the answer.

First Word Containing a Prefix

Algorithms & Data Structures · lower_bound on the prefix itself — the follow-ups are the round.

Stateful Search Autocomplete Session

Algorithms & Data Structures · LC 642: a typing session over historical search frequencies.

Reverse Count-and-Say

Algorithms & Data Structures · Parse (count, digit) pairs backward — adjacent runs must differ.