Coding problems by concept

All 56 existing problems have one home. Start with the linked primer, attempt the problem before opening its answer, and use its follow-ups to change the contract. The number is a stable problem identifier, not a seniority level.

Data structures and algorithms · Timed assessment

Collections, windows and summaries

ID	Problem	Subject home
01	Two sum: remember the useful past	Data structures and algorithms
02	Valid anagram: equality of multiplicities	Data structures and algorithms
03	Group anagrams: canonical keys	Data structures and algorithms
04	Longest unique window: move the boundary forward	Data structures and algorithms
05	Minimum covering window: track unmet demand	Data structures and algorithms
06	Count target-sum subarrays: differences of prefixes	Data structures and algorithms
07	Product except self: combine independent summaries	Data structures and algorithms
08	Longest consecutive run: expand only from starts	Data structures and algorithms

Ordering and boundary search

ID	Problem	Subject home
09	Merge intervals: preserve the covered set	Data structures and algorithms
10	Meeting room capacity: count simultaneous demand	Data structures and algorithms
11	Binary search boundary: find the first true position	Data structures and algorithms
12	Rotated array search: identify the ordered half	Data structures and algorithms
13	Shipping capacity: search a feasible answer	Data structures and algorithms

Pointers and trees

ID	Problem	Subject home
14	Reverse a linked list: keep the unprocessed suffix reachable	Data structures and algorithms
15	Find where a linked list loops back on itself	Data structures and algorithms
16	Merge sorted lists: splice only a safe frontier	Data structures and algorithms
17	Tree level order: keep the next frontier separate	Data structures and algorithms
18	Validate a BST: carry every ancestor constraint	Data structures and algorithms
19	Lowest common ancestor: return presence as well as a candidate	Data structures and algorithms
20	Tree diameter: return one branch, combine two locally	Data structures and algorithms

Graphs and heaps

ID	Problem	Subject home
21	Dependency order	Data structures and algorithms
22	Word ladder	Data structures and algorithms
23	Connectivity under added links	Data structures and algorithms
24	Shortest path through a grid	Data structures and algorithms
25	Cheapest route with nonnegative costs	Data structures and algorithms
26	Top k observations in a stream	Data structures and algorithms
27	Merge k sorted streams	Data structures and algorithms

Caches and prefix structures

ID	Problem	Subject home
28	Implement LRU without an ordered-map helper	Data systems at scale
29	Expiring key-value store	Data systems at scale
30	Prefix autocomplete	Data structures and algorithms

Search and dynamic programming

ID	Problem	Subject home
31	Search for unique combinations	Data structures and algorithms
32	Find a word without reusing a cell	Data structures and algorithms
33	Minimum coins with a witness	Data structures and algorithms
34	Longest increasing subsequence	Data structures and algorithms
35	Edit distance	Data structures and algorithms
36	Count valid digit decodings	Data structures and algorithms

Stacks, parsing, streams and synchronization

ID	Problem	Subject home
37	Days until a warmer temperature	Data structures and algorithms
38	Largest rectangle in a histogram	Data structures and algorithms
39	Parse and evaluate a policy expression	Data structures and algorithms
40	Count event-time windows with late arrivals	Data systems at scale
41	Maintain an exact streaming median	Data structures and algorithms
42	Bounded blocking queue with shutdown	APIs and background work

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