Design LeetCode Problems: All 122,
With Python Solutions

Every problem in this library that LeetCode tags Design. 89 of them carry a complete Python solution, a worked example and the time and space complexity of the approach.

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Easy: 13
Medium: 79
Hard: 30

Design problems by difficulty

Problems with a complete Python solution are listed first, then by ascending problem number.

Easy (13)

Easy problems
No.	Problem	Topics
0225	Implement Stack using Queues	Stack, Design, Queue
0232	Implement Queue using Stacks	Stack, Design, Queue
0303	Range Sum Query - Immutable	Design, Array, Prefix Sum
0703	Kth Largest Element in a Stream	Tree, Design, Binary Search Tree +3
0705	Design HashSet	Design, Array, Hash Table +2
0706	Design HashMap	Design, Array, Hash Table +2
0933	Number of Recent Calls	Design, Queue, Data Stream
1603	Design Parking System	Design, Counting, Simulation
1656	Design an Ordered Stream	Design, Array, Hash Table +1
0170	Two Sum III - Data structure designPremium	Design, Array, Hash Table +2
0346	Moving Average from Data StreamPremium	Design, Queue, Array +1
0359	Logger Rate LimiterPremium	Design, Hash Table, Data Stream
0604	Design Compressed String IteratorPremium	Design, Array, String +1

Medium (79)

Medium problems
No.	Problem	Topics
0146	LRU Cache	Design, Hash Table, Linked List +1
0155	Min Stack	Stack, Design
0173	Binary Search Tree Iterator	Stack, Tree, Design +3
0208	Implement Trie (Prefix Tree)	Design, Trie, Hash Table +1
0211	Design Add and Search Words Data Structure	Depth-First Search, Design, Trie +1
0284	Peeking Iterator	Design, Array, Iterator
0304	Range Sum Query 2D - Immutable	Design, Array, Matrix +1
0307	Range Sum Query - Mutable	Design, Binary Indexed Tree, Segment Tree +2
0341	Flatten Nested List Iterator	Stack, Tree, Depth-First Search +3
0355	Design Twitter	Design, Hash Table, Linked List +1
0380	Insert Delete GetRandom O(1)	Design, Array, Hash Table +2
0384	Shuffle an Array	Design, Array, Math +1
0449	Serialize and Deserialize BST	Tree, Depth-First Search, Breadth-First Search +4
0535	Encode and Decode TinyURL	Design, Hash Table, String +1
0622	Design Circular Queue	Design, Queue, Array +1
0641	Design Circular Deque	Design, Queue, Array +1
0676	Implement Magic Dictionary	Depth-First Search, Design, Trie +2
0677	Map Sum Pairs	Design, Trie, Hash Table +1
0707	Design Linked List	Design, Linked List
0729	My Calendar I	Design, Segment Tree, Array +2
0731	My Calendar II	Design, Segment Tree, Array +3
0855	Exam Room	Design, Ordered Set, Heap (Priority Queue)
0900	RLE Iterator	Design, Array, Counting +1
0901	Online Stock Span	Stack, Design, Data Stream +1
0911	Online Election	Design, Array, Hash Table +1
0919	Complete Binary Tree Inserter	Tree, Breadth-First Search, Design +1
0981	Time Based Key-Value Store	Design, Hash Table, String +1
1146	Snapshot Array	Design, Array, Hash Table +1
1261	Find Elements in a Contaminated Binary Tree	Tree, Depth-First Search, Breadth-First Search +3
1286	Iterator for Combination	Design, String, Backtracking +1
1348	Tweet Counts Per Frequency	Design, Hash Table, String +3
1352	Product of the Last K Numbers	Design, Array, Math +2
1357	Apply Discount Every n Orders	Design, Array, Hash Table
1381	Design a Stack With Increment Operation	Stack, Design, Array
1396	Design Underground System	Design, Hash Table, String
1472	Design Browser History	Stack, Design, Array +3
1476	Subrectangle Queries	Design, Array, Matrix
1600	Throne Inheritance	Tree, Depth-First Search, Design +1
1670	Design Front Middle Back Queue	Design, Queue, Array +2
1797	Design Authentication Manager	Design, Hash Table, Linked List +1
1845	Seat Reservation Manager	Design, Heap (Priority Queue)
1865	Finding Pairs With a Certain Sum	Design, Array, Hash Table
1993	Operations on Tree	Tree, Depth-First Search, Breadth-First Search +3
2013	Detect Squares	Design, Array, Hash Table +1
2034	Stock Price Fluctuation	Design, Hash Table, Data Stream +2
2043	Simple Bank System	Design, Array, Hash Table +1
2080	Range Frequency Queries	Design, Segment Tree, Array +2
2166	Design Bitset	Design, Array, Hash Table +1
2241	Design an ATM Machine	Greedy, Design, Array
2336	Smallest Number in Infinite Set	Design, Hash Table, Ordered Set +1
2349	Design a Number Container System	Design, Hash Table, Ordered Set +1
2353	Design a Food Rating System	Design, Array, Hash Table +3
2424	Longest Uploaded Prefix	Union Find, Design, Binary Indexed Tree +5
2502	Design Memory Allocator	Design, Array, Hash Table +1
2526	Find Consecutive Integers from a Data Stream	Design, Queue, Hash Table +2
2671	Frequency Tracker	Design, Hash Table
0244	Shortest Word Distance IIPremium	Design, Array, Hash Table +2
0251	Flatten 2D VectorPremium	Design, Array, Two Pointers +1
0271	Encode and Decode StringsPremium	Design, Array, String
0281	Zigzag IteratorPremium	Design, Queue, Array +1
0288	Unique Word AbbreviationPremium	Design, Array, Hash Table +1
0308	Range Sum Query 2D - MutablePremium	Design, Binary Indexed Tree, Segment Tree +2
0348	Design Tic-Tac-ToePremium	Design, Array, Hash Table +2
0353	Design Snake GamePremium	Design, Queue, Array +2
0362	Design Hit CounterPremium	Design, Queue, Array +2
0379	Design Phone DirectoryPremium	Design, Queue, Array +2
0635	Design Log Storage SystemPremium	Design, Hash Table, String +1
1166	Design File SystemPremium	Design, Trie, Hash Table +1
1244	Design A LeaderboardPremium	Design, Hash Table, Sorting
1429	First Unique NumberPremium	Design, Queue, Array +2
1500	Design a File Sharing SystemPremium	Design, Hash Table, Data Stream +2
1570	Dot Product of Two Sparse VectorsPremium	Design, Array, Hash Table +1
1586	Binary Search Tree Iterator IIPremium	Stack, Tree, Design +3
1628	Design an Expression Tree With Evaluate FunctionPremium	Stack, Tree, Design +3
1756	Design Most Recently Used QueuePremium	Design, Array, Linked List +3
1804	Implement Trie II (Prefix Tree)Premium	Design, Trie, Hash Table +1
2069	Walking Robot Simulation II	Design, Simulation
2408	Design SQLPremium	Design, Array, Hash Table +1
2590	Design a Todo ListPremium	Design, Array, Hash Table +2

Hard (30)

Hard problems
No.	Problem	Topics
0295	Find Median from Data Stream	Design, Two Pointers, Data Stream +2
0297	Serialize and Deserialize Binary Tree	Tree, Depth-First Search, Breadth-First Search +3
0352	Data Stream as Disjoint Intervals	Union Find, Design, Hash Table +3
0381	Insert Delete GetRandom O(1) - Duplicates allowed	Design, Array, Hash Table +2
0432	All O`one Data Structure	Design, Hash Table, Linked List +1
0460	LFU Cache	Design, Hash Table, Linked List +1
0715	Range Module	Design, Segment Tree, Ordered Set
0732	My Calendar III	Design, Segment Tree, Binary Search +2
0745	Prefix and Suffix Search	Design, Trie, Array +2
0895	Maximum Frequency Stack	Stack, Design, Hash Table +1
1032	Stream of Characters	Design, Trie, Array +2
1157	Online Majority Element In Subarray	Design, Binary Indexed Tree, Segment Tree +2
1172	Dinner Plate Stacks	Stack, Design, Hash Table +1
1206	Design Skiplist	Design, Linked List
1483	Kth Ancestor of a Tree Node	Bit Manipulation, Tree, Depth-First Search +4
1622	Fancy Sequence	Design, Segment Tree, Math
1825	Finding MK Average	Design, Queue, Data Stream +2
1912	Design Movie Rental System	Design, Array, Hash Table +2
2102	Sequentially Ordinal Rank Tracker	Design, Data Stream, Ordered Set +1
2227	Encrypt and Decrypt Strings	Design, Trie, Array +2
2276	Count Integers in Intervals	Design, Segment Tree, Ordered Set
2286	Booking Concert Tickets in Groups	Design, Binary Indexed Tree, Segment Tree +1
2296	Design a Text Editor	Stack, Design, Linked List +3
2642	Design Graph With Shortest Path Calculator	Graph, Design, Shortest Path +1
0431	Encode N-ary Tree to Binary TreePremium	Tree, Depth-First Search, Breadth-First Search +2
0588	Design In-Memory File SystemPremium	Design, Trie, Hash Table +2
0631	Design Excel Sum FormulaPremium	Graph, Design, Topological Sort +4
0642	Design Search Autocomplete SystemPremium	Depth-First Search, Design, Trie +4
0716	Max StackPremium	Stack, Design, Linked List +2
2254	Design Video Sharing PlatformPremium	Stack, Design, Hash Table +1

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python

Please analyze this context and help me answer the question or solve the problem.

Technical interview · 45 minsolution.pyShared editor · Python 3Longest substring without repeatsMediumGiven a string s, return the length of the longest contiguous substring in which every character is unique.s = "abcabcbb"
→ 31def length_of_longest_substring(s: str) -> int:2 last_seen = {}3 left = best = 04 for right, ch in enumerate(s):5 if ch in last_seen and last_seen[ch] >= left:

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

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GPT-6.1 Terra

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python

Please analyze this context and help me answer the question or solve the problem.

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

Type your message...

GPT-6.1 TerraClear chat

QuitEscHide+QCapture+PCrop+OSubmit+EnterPress to get answerLeft/Right+Up/Down+

90%Subscription

python

Please analyze this context and help me answer the question or solve the problem.

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

Type your message...

GPT-6.1 TerraClear chat

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QuitEscHide+QCapture+PCrop+OSubmit+EnterPress to get answerLeft/Right+Up/Down+

Design problems by difficulty

Easy (13)13

Medium (79)79

Hard (30)30

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Easy (13)

Medium (79)

Hard (30)