Data Structures & Algorithms Interview Questions: Basic to Advanced

1What is Big-O, Big-Theta, Big-Omega, and what problem does it solve?Basicconceptual30 minIn writing
2Walk me through how time vs space complexity actually works under the hood.Basicconceptual35 minIn writing
3What's the practical difference that trips people up about amortized analysis (e.g. dynamic array growth)?Basicconceptual30 minIn writing
4How would you use best/average/worst case in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
5What's the tradeoff with dropping constants and lower-order terms, and when would you choose differently?Intermediatepractical35 minIn writing
6What's a subtle bug or edge case people hit with common complexity classes ranked?Intermediateconceptual30 minIn writing
7In production, an issue traces back to big-o, big-theta, big-omega. How do you diagnose and fix it?Scenarioscenario45 minIn writing
8Quiz: check your grasp of big-o and complexity analysis.Basicconceptual20 minIn writing

9What is Two-pointer technique, and what problem does it solve?Basicconceptual30 minIn writing
10Walk me through how sliding window pattern actually works under the hood.Basicconceptual35 minIn writing
11What's the practical difference that trips people up about in-place array manipulation?Basicconceptual30 minIn writing
12How would you use string immutability's cost in loops in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
13What's the tradeoff with prefix sums, and when would you choose differently?Intermediatepractical35 minIn writing
14What's a subtle bug or edge case people hit with common off-by-one pitfalls?Intermediateconceptual30 minIn writing
15In production, an issue traces back to two-pointer technique. How do you diagnose and fix it?Scenarioscenario45 minIn writing
16Quiz: check your grasp of arrays and strings.Basicconceptual20 minIn writing

17What is Singly vs doubly linked lists, and what problem does it solve?Basicconceptual30 minIn writing
18Walk me through how reversing a linked list actually works under the hood.Basicconceptual35 minIn writing
19What's the practical difference that trips people up about fast/slow pointer cycle detection?Basicconceptual30 minIn writing
20How would you use dummy head node technique in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
21What's the tradeoff with merging sorted lists, and when would you choose differently?Intermediatepractical35 minIn writing
22What's a subtle bug or edge case people hit with when an array beats a linked list?Intermediateconceptual30 minIn writing
23In production, an issue traces back to singly vs doubly linked lists. How do you diagnose and fix it?Scenarioscenario45 minIn writing
24Quiz: check your grasp of linked lists.Basicconceptual20 minIn writing

25What is Stack via array vs linked list, and what problem does it solve?Basicconceptual30 minIn writing
26Walk me through how queue via two stacks actually works under the hood.Basicconceptual35 minIn writing
27What's the practical difference that trips people up about monotonic stack pattern?Basicconceptual30 minIn writing
28How would you use circular buffer for a bounded queue in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
29What's the tradeoff with deque and its use cases, and when would you choose differently?Intermediatepractical35 minIn writing
30What's a subtle bug or edge case people hit with call stack as an implicit stack?Intermediateconceptual30 minIn writing
31In production, an issue traces back to stack via array vs linked list. How do you diagnose and fix it?Scenarioscenario45 minIn writing
32Quiz: check your grasp of stacks and queues.Basicconceptual20 minIn writing

33What is Hashing and collision resolution, and what problem does it solve?Basicconceptual30 minIn writing
34Walk me through how chaining vs open addressing actually works under the hood.Basicconceptual35 minIn writing
35What's the practical difference that trips people up about load factor and resizing?Basicconceptual30 minIn writing
36How would you use when hash tables degrade to o(n) in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
37What's the tradeoff with using a hash set for dedup patterns, and when would you choose differently?Intermediatepractical35 minIn writing
38What's a subtle bug or edge case people hit with designing a hashable key from a composite value?Intermediateconceptual30 minIn writing
39In production, an issue traces back to hashing and collision resolution. How do you diagnose and fix it?Scenarioscenario45 minIn writing
40Quiz: check your grasp of hash tables.Basicconceptual20 minIn writing

41What is Tree traversals: pre/in/post/level order, and what problem does it solve?Basicconceptual30 minIn writing
42Walk me through how bst invariant and why it can degrade actually works under the hood.Basicconceptual35 minIn writing
43What's the practical difference that trips people up about balanced trees (avl, red-black) at a glance?Basicconceptual30 minIn writing
44How would you use lowest common ancestor in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
45What's the tradeoff with tree height vs balance, and when would you choose differently?Intermediatepractical35 minIn writing
46What's a subtle bug or edge case people hit with recursive vs iterative traversal?Intermediateconceptual30 minIn writing
47In production, an issue traces back to tree traversals: pre/in/post/level order. How do you diagnose and fix it?Scenarioscenario45 minIn writing
48Quiz: check your grasp of trees and binary search trees.Basicconceptual20 minIn writing

49What is Binary heap array representation, and what problem does it solve?Basicconceptual30 minIn writing
50Walk me through how heapify up/down actually works under the hood.Basicconceptual35 minIn writing
51What's the practical difference that trips people up about min-heap vs max-heap use cases?Basicconceptual30 minIn writing
52How would you use top-k problems with a heap in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
53What's the tradeoff with heap vs sorted array tradeoffs, and when would you choose differently?Intermediatepractical35 minIn writing
54What's a subtle bug or edge case people hit with building a heap in o(n)?Intermediateconceptual30 minIn writing
55In production, an issue traces back to binary heap array representation. How do you diagnose and fix it?Scenarioscenario45 minIn writing
56Quiz: check your grasp of heaps and priority queues.Basicconceptual20 minIn writing

57What is Adjacency list vs adjacency matrix, and what problem does it solve?Basicconceptual30 minIn writing
58Walk me through how bfs vs dfs and when each applies actually works under the hood.Basicconceptual35 minIn writing
59What's the practical difference that trips people up about detecting a cycle?Basicconceptual30 minIn writing
60How would you use connected components in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
61What's the tradeoff with directed vs undirected graph traversal, and when would you choose differently?Intermediatepractical35 minIn writing
62What's a subtle bug or edge case people hit with topological sort prerequisites?Intermediateconceptual30 minIn writing
63In production, an issue traces back to adjacency list vs adjacency matrix. How do you diagnose and fix it?Scenarioscenario45 minIn writing
64Quiz: check your grasp of graphs: representation and traversal.Basicconceptual20 minIn writing

65What is Comparison sorts: merge, quick, heap, and what problem does it solve?Basicconceptual30 minIn writing
66Walk me through how stability and why it matters actually works under the hood.Basicconceptual35 minIn writing
67What's the practical difference that trips people up about in-place vs extra-memory sorts?Basicconceptual30 minIn writing
68How would you use non-comparison sorts: counting, radix in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
69What's the tradeoff with choosing a sort for nearly-sorted data, and when would you choose differently?Intermediatepractical35 minIn writing
70What's a subtle bug or edge case people hit with why quicksort's worst case is o(n²)?Intermediateconceptual30 minIn writing
71In production, an issue traces back to comparison sorts: merge, quick, heap. How do you diagnose and fix it?Scenarioscenario45 minIn writing
72Quiz: check your grasp of sorting algorithms.Basicconceptual20 minIn writing

73What is Classic binary search invariants, and what problem does it solve?Basicconceptual30 minIn writing
74Walk me through how search in rotated sorted array actually works under the hood.Basicconceptual35 minIn writing
75What's the practical difference that trips people up about binary search on the answer space?Basicconceptual30 minIn writing
76How would you use finding boundaries (first/last occurrence) in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
77What's the tradeoff with off-by-one bugs in binary search, and when would you choose differently?Intermediatepractical35 minIn writing
78What's a subtle bug or edge case people hit with when linear search is still correct to choose?Intermediateconceptual30 minIn writing
79In production, an issue traces back to classic binary search invariants. How do you diagnose and fix it?Scenarioscenario45 minIn writing
80Quiz: check your grasp of searching and binary search variants.Basicconceptual20 minIn writing

81What is Base case and recursive case design, and what problem does it solve?Basicconceptual30 minIn writing
82Walk me through how recursion vs iteration tradeoffs actually works under the hood.Basicconceptual35 minIn writing
83What's the practical difference that trips people up about backtracking template (choose/explore/unchoose)?Basicconceptual30 minIn writing
84How would you use pruning a backtracking search in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
85What's the tradeoff with recursion depth and stack overflow risk, and when would you choose differently?Intermediatepractical35 minIn writing
86What's a subtle bug or edge case people hit with converting recursion to iteration with an explicit stack?Intermediateconceptual30 minIn writing
87In production, an issue traces back to base case and recursive case design. How do you diagnose and fix it?Scenarioscenario45 minIn writing
88Quiz: check your grasp of recursion and backtracking.Basicconceptual20 minIn writing

89What is Overlapping subproblems and optimal substructure, and what problem does it solve?Basicconceptual30 minIn writing
90Walk me through how memoization (top-down) vs tabulation (bottom-up) actually works under the hood.Basicconceptual35 minIn writing
91What's the practical difference that trips people up about identifying a dp problem?Basicconceptual30 minIn writing
92How would you use 1d dp: climbing stairs, house robber style in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
93What's the tradeoff with 2d dp: grid paths, edit distance style, and when would you choose differently?Intermediatepractical35 minIn writing
94What's a subtle bug or edge case people hit with space-optimizing a dp table?Intermediateconceptual30 minIn writing
95In production, an issue traces back to overlapping subproblems and optimal substructure. How do you diagnose and fix it?Scenarioscenario45 minIn writing
96Quiz: check your grasp of dynamic programming fundamentals.Basicconceptual20 minIn writing

97What is 0/1 knapsack pattern, and what problem does it solve?Basicconceptual30 minIn writing
98Walk me through how unbounded knapsack pattern actually works under the hood.Basicconceptual35 minIn writing
99What's the practical difference that trips people up about interval dp (matrix chain style)?Basicconceptual30 minIn writing
100How would you use state-machine dp (buy/sell stock style) in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
101What's the tradeoff with dp on trees, and when would you choose differently?Intermediatepractical35 minIn writing
102What's a subtle bug or edge case people hit with bitmask dp for small state spaces?Intermediateconceptual30 minIn writing
103In production, an issue traces back to 0/1 knapsack pattern. How do you diagnose and fix it?Scenarioscenario45 minIn writing
104Quiz: check your grasp of advanced dp patterns.Basicconceptual20 minIn writing

105What is Greedy vs DP: when greedy actually works, and what problem does it solve?Basicconceptual30 minIn writing
106Walk me through how interval scheduling actually works under the hood.Basicconceptual35 minIn writing
107What's the practical difference that trips people up about proving greedy correctness (exchange argument)?Basicconceptual30 minIn writing
108How would you use huffman coding as a greedy example in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
109What's the tradeoff with greedy on graphs (mst intuition), and when would you choose differently?Intermediatepractical35 minIn writing
110What's a subtle bug or edge case people hit with common greedy traps that look correct?Intermediateconceptual30 minIn writing
111In production, an issue traces back to greedy vs dp: when greedy actually works. How do you diagnose and fix it?Scenarioscenario45 minIn writing
112Quiz: check your grasp of greedy algorithms.Basicconceptual20 minIn writing

113What is Trie structure and insertion, and what problem does it solve?Basicconceptual30 minIn writing
114Walk me through how prefix search and autocomplete actually works under the hood.Basicconceptual35 minIn writing
115What's the practical difference that trips people up about string matching (kmp intuition)?Basicconceptual30 minIn writing
116How would you use palindrome-related patterns in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
117What's the tradeoff with anagram detection via sorting or counting, and when would you choose differently?Intermediatepractical35 minIn writing
118What's a subtle bug or edge case people hit with space cost of a trie vs a hash set?Intermediateconceptual30 minIn writing
119In production, an issue traces back to trie structure and insertion. How do you diagnose and fix it?Scenarioscenario45 minIn writing
120Quiz: check your grasp of tries and string algorithms.Basicconceptual20 minIn writing

121What is Designing an LRU cache, and what problem does it solve?Basicconceptual30 minIn writing
122Walk me through how designing a rate limiter actually works under the hood.Basicconceptual35 minIn writing
123What's the practical difference that trips people up about combining a hash map with a linked list?Basicconceptual30 minIn writing
124How would you use amortized complexity in a designed structure in a real implementation, and what would you watch out for?Intermediatepractical35 minIn writing
125What's the tradeoff with thread-safety considerations in a designed structure, and when would you choose differently?Intermediatepractical35 minIn writing
126What's a subtle bug or edge case people hit with talking through tradeoffs out loud?Intermediateconceptual30 minIn writing
127In production, an issue traces back to designing an lru cache. How do you diagnose and fix it?Scenarioscenario45 minIn writing
128Quiz: check your grasp of system-style algorithm design.Basicconceptual20 minIn writing