Free · no solutions
Hints, not answers
Three hints per problem, from a nudge to the approach. In Unaided they unlock at 8, 15 and 20 minutes. Here you choose when, but try eight minutes on your own first.
Arrays & hashing
- First rescan Easy
- Shortfall report Easy
- Bay switchers Medium
- Dispatch run with one holiday Medium
- Peak hour per zone Medium
- Tightest rescan Medium
- Sole-source parts Medium
- Even-flow stretch Hard
Two pointers
- Silence trim Easy
- Reconciled event log Easy
- Chord within a span Medium
- Nearest marker pair Medium
- Pairs under budget Medium
- Alternating polarity Medium
- Smallest re-edit span Medium
- Tight three-way take Hard
Sliding window
- Peak k-hour draw Easy
- Spike windows Easy
- Longest under-budget stretch Medium
- Meter variety windows Medium
- Full tariff coverage Medium
- Steady stretch Medium
- One surge allowed Medium
- Discounted peak stretch Hard
Stack
- Tag balance Easy
- Undo counter Easy
- Distance to next dedent Medium
- Nested comment stripper Medium
- Outline numbering Medium
- Prefix formula evaluator Medium
- Undo-redo buffer Medium
- Longest clean markup run Hard
Binary search
- Smallest sufficient tier Easy
- Leaderboard rank Easy
- Largest tile size Medium
- Requests in a window Medium
- Cheapest point on the curve Medium
- Earliest time for k jobs Medium
- Fair cap Medium
- Kth smallest metered bill Hard
Linked list
- Track at offset Easy
- Drop skipped tracks Easy
- Alternate merge Medium
- Split into rooms Medium
- Remove repeated tracks entirely Medium
- Split by rating Medium
- Insert into a loop Medium
- Setlist rank tracker Hard
Trees
- Team size Easy
- Individual contributors Easy
- Leftmost desk on the lowest floor Medium
- Longest same-role chain Medium
- Oversized folders Medium
- Imbalance score Medium
- Common manager Medium
- Cheapest on-call cover Hard
Heap / priority queue
- Serve by urgency Easy
- Top open tickets Easy
- Last call cleared Medium
- Deadline-limited repairs Medium
- First n across logs Medium
- Live top-k total Medium
- Blend to threshold Medium
- Median of the open board Hard
Backtracking
- Full menus Easy
- Lucky tickets Easy
- Conflict-free timetables Medium
- Peaceful seating Medium
- Secret gift assignments Medium
- Stall layouts Medium
- Zone colouring Medium
- Ballroom floor covering Hard
Graphs
- Busiest interchange Easy
- Dead-end stations Easy
- Fewest line changes Medium
- Earliest arrival Medium
- Two-operator split Medium
- Reachable after a closure Medium
- Distinct fastest routes Medium
- Single points of failure Hard
Tries
- Shared path root Easy
- Words under a prefix Easy
- Suggest as you type Medium
- Wildcard routes Medium
- Shortest unique prefix Medium
- Buildable name Medium
- Prefix-scoped store Medium
- One-slip search Hard
Intervals
- Clash finder Easy
- Booked minutes Easy
- Free gaps Medium
- First joint slot Medium
- Shared opening hours Medium
- Uncovered bookings Medium
- Fewest announcements Medium
- Fewest cancellations under capacity Hard
Greedy
- Most crates loaded Easy
- Fewest bills Easy
- Deadline sequencing Medium
- Best after k flips Medium
- Unique stock counts Medium
- Fewest sprinklers Medium
- Flexible brackets Medium
- Cheapest refuel plan Hard
1-D dynamic programming
- Batch growth Easy
- Best sales run Easy
- Cut the dough Medium
- Oven load Medium
- Longest zigzag sales run Medium
- Bulk discount picks Medium
- Cheapest pass plan Medium
- Cheapest prep schedule Hard
2-D dynamic programming
- Rectangle totals Easy
- Binomial bouquets Easy
- Longest shared run Medium
- Fewest deletions to match Medium
- Wall-bounce paths Medium
- Largest plus bed Medium
- Two-resource pack Medium
- Cheapest version merge Hard