Medium LeetCode Problems: All 1,579,
With Python Solutions

Medium is where most interview questions actually sit. The technique is no longer stated — recognising which pattern applies is the problem, and the implementation is the easy half once you have.

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The patterns behind medium problems

The six patterns with the most Medium-rated problems in this library. Working through a pattern beats working down a difficulty list, because the technique transfers and a difficulty band does not.

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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

Primary display

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

Page 1 of 8

Medium problems

Showing the first 200 of 1,579 problems. Problems with a complete Python solution are listed first, then by ascending problem number.

Medium problems
No.	Problem	Topics
0002	Add Two Numbers	Recursion, Linked List, Math
0003	Longest Substring Without Repeating Characters	Hash Table, String, Sliding Window
0005	Longest Palindromic Substring	Two Pointers, String, Dynamic Programming
0006	Zigzag Conversion	String
0007	Reverse Integer	Math
0008	String to Integer (atoi)	String
0011	Container With Most Water	Greedy, Array, Two Pointers
0012	Integer to Roman	Hash Table, Math, String
0015	3Sum	Array, Two Pointers, Sorting
0016	3Sum Closest	Array, Two Pointers, Sorting
0017	Letter Combinations of a Phone Number	Hash Table, String, Backtracking
0018	4Sum	Array, Two Pointers, Sorting
0019	Remove Nth Node From End of List	Linked List, Two Pointers
0022	Generate Parentheses	String, Dynamic Programming, Backtracking
0024	Swap Nodes in Pairs	Recursion, Linked List
0029	Divide Two Integers	Bit Manipulation, Math
0031	Next Permutation	Array, Two Pointers
0033	Search in Rotated Sorted Array	Array, Binary Search
0034	Find First and Last Position of Element in Sorted Array	Array, Binary Search
0036	Valid Sudoku	Array, Hash Table, Matrix
0038	Count and Say	String
0039	Combination Sum	Array, Backtracking
0040	Combination Sum II	Array, Backtracking
0043	Multiply Strings	Math, String, Simulation
0045	Jump Game II	Greedy, Array, Dynamic Programming
0046	Permutations	Array, Backtracking
0047	Permutations II	Array, Backtracking, Sorting
0048	Rotate Image	Array, Math, Matrix
0049	Group Anagrams	Array, Hash Table, String +1
0050	Pow(x, n)	Recursion, Math
0053	Maximum Subarray	Array, Divide and Conquer, Dynamic Programming
0054	Spiral Matrix	Array, Matrix, Simulation
0055	Jump Game	Greedy, Array, Dynamic Programming
0056	Merge Intervals	Array, Sorting
0057	Insert Interval	Array
0059	Spiral Matrix II	Array, Matrix, Simulation
0061	Rotate List	Linked List, Two Pointers
0062	Unique Paths	Math, Dynamic Programming, Combinatorics
0063	Unique Paths II	Array, Dynamic Programming, Matrix
0064	Minimum Path Sum	Array, Dynamic Programming, Matrix
0071	Simplify Path	Stack, String
0072	Edit Distance	String, Dynamic Programming
0073	Set Matrix Zeroes	Array, Hash Table, Matrix
0074	Search a 2D Matrix	Array, Binary Search, Matrix
0075	Sort Colors	Array, Two Pointers, Sorting
0077	Combinations	Backtracking
0078	Subsets	Bit Manipulation, Array, Backtracking
0079	Word Search	Depth-First Search, Array, String +2
0080	Remove Duplicates from Sorted Array II	Array, Two Pointers
0081	Search in Rotated Sorted Array II	Array, Binary Search
0082	Remove Duplicates from Sorted List II	Linked List, Two Pointers
0086	Partition List	Linked List, Two Pointers
0089	Gray Code	Bit Manipulation, Math, Backtracking
0090	Subsets II	Bit Manipulation, Array, Backtracking
0091	Decode Ways	String, Dynamic Programming
0092	Reverse Linked List II	Linked List
0093	Restore IP Addresses	String, Backtracking
0095	Unique Binary Search Trees II	Tree, Binary Search Tree, Dynamic Programming +2
0096	Unique Binary Search Trees	Tree, Binary Search Tree, Math +2
0097	Interleaving String	String, Dynamic Programming
0098	Validate Binary Search Tree	Tree, Depth-First Search, Binary Search Tree +1
0099	Recover Binary Search Tree	Tree, Depth-First Search, Binary Search Tree +1
0102	Binary Tree Level Order Traversal	Tree, Breadth-First Search, Binary Tree
0103	Binary Tree Zigzag Level Order Traversal	Tree, Breadth-First Search, Binary Tree
0105	Construct Binary Tree from Preorder and Inorder Traversal	Tree, Array, Hash Table +2
0106	Construct Binary Tree from Inorder and Postorder Traversal	Tree, Array, Hash Table +2
0107	Binary Tree Level Order Traversal II	Tree, Breadth-First Search, Binary Tree
0109	Convert Sorted List to Binary Search Tree	Tree, Binary Search Tree, Linked List +2
0113	Path Sum II	Tree, Depth-First Search, Backtracking +1
0114	Flatten Binary Tree to Linked List	Stack, Tree, Depth-First Search +2
0116	Populating Next Right Pointers in Each Node	Tree, Depth-First Search, Breadth-First Search +2
0117	Populating Next Right Pointers in Each Node II	Tree, Depth-First Search, Breadth-First Search +2
0120	Triangle	Array, Dynamic Programming
0122	Best Time to Buy and Sell Stock II	Greedy, Array, Dynamic Programming
0128	Longest Consecutive Sequence	Union Find, Array, Hash Table
0129	Sum Root to Leaf Numbers	Tree, Depth-First Search, Binary Tree
0130	Surrounded Regions	Depth-First Search, Breadth-First Search, Union Find +2
0131	Palindrome Partitioning	String, Dynamic Programming, Backtracking
0133	Clone Graph	Depth-First Search, Breadth-First Search, Graph +1
0134	Gas Station	Greedy, Array
0137	Single Number II	Bit Manipulation, Array
0138	Copy List with Random Pointer	Hash Table, Linked List
0139	Word Break	Trie, Memoization, Array +3
0142	Linked List Cycle II	Hash Table, Linked List, Two Pointers
0143	Reorder List	Stack, Recursion, Linked List +1
0146	LRU Cache	Design, Hash Table, Linked List +1
0147	Insertion Sort List	Linked List, Sorting
0148	Sort List	Linked List, Two Pointers, Divide and Conquer +2
0150	Evaluate Reverse Polish Notation	Stack, Array, Math
0151	Reverse Words in a String	Two Pointers, String
0152	Maximum Product Subarray	Array, Dynamic Programming
0153	Find Minimum in Rotated Sorted Array	Array, Binary Search
0155	Min Stack	Stack, Design
0162	Find Peak Element	Array, Binary Search
0164	Maximum Gap	Array, Bucket Sort, Radix Sort +1
0165	Compare Version Numbers	Two Pointers, String
0166	Fraction to Recurring Decimal	Hash Table, Math, String
0167	Two Sum II - Input Array Is Sorted	Array, Two Pointers, Binary Search
0172	Factorial Trailing Zeroes	Math
0173	Binary Search Tree Iterator	Stack, Tree, Design +3
0176	Second Highest Salary	Database
0177	Nth Highest Salary	Database
0178	Rank Scores	Database
0179	Largest Number	Greedy, Array, String +1
0180	Consecutive Numbers	Database
0184	Department Highest Salary	Database
0187	Repeated DNA Sequences	Bit Manipulation, Hash Table, String +3
0189	Rotate Array	Array, Math, Two Pointers
0192	Word Frequency	Shell
0194	Transpose File	Shell
0198	House Robber	Array, Dynamic Programming
0199	Binary Tree Right Side View	Tree, Depth-First Search, Breadth-First Search +1
0200	Number of Islands	Depth-First Search, Breadth-First Search, Union Find +2
0201	Bitwise AND of Numbers Range	Bit Manipulation
0204	Count Primes	Array, Math, Enumeration +1
0207	Course Schedule	Depth-First Search, Breadth-First Search, Graph +1
0208	Implement Trie (Prefix Tree)	Design, Trie, Hash Table +1
0209	Minimum Size Subarray Sum	Array, Binary Search, Prefix Sum +1
0210	Course Schedule II	Depth-First Search, Breadth-First Search, Graph +1
0211	Design Add and Search Words Data Structure	Depth-First Search, Design, Trie +1
0213	House Robber II	Array, Dynamic Programming
0215	Kth Largest Element in an Array	Array, Divide and Conquer, Quickselect +2
0216	Combination Sum III	Array, Backtracking
0221	Maximal Square	Array, Dynamic Programming, Matrix
0223	Rectangle Area	Geometry, Math
0227	Basic Calculator II	Stack, Math, String
0229	Majority Element II	Array, Hash Table, Counting +1
0230	Kth Smallest Element in a BST	Tree, Depth-First Search, Binary Search Tree +1
0235	Lowest Common Ancestor of a Binary Search Tree	Tree, Depth-First Search, Binary Search Tree +1
0236	Lowest Common Ancestor of a Binary Tree	Tree, Depth-First Search, Binary Tree
0237	Delete Node in a Linked List	Linked List
0238	Product of Array Except Self	Array, Prefix Sum
0240	Search a 2D Matrix II	Array, Binary Search, Divide and Conquer +1
0241	Different Ways to Add Parentheses	Recursion, Memoization, Math +2
0260	Single Number III	Bit Manipulation, Array
0264	Ugly Number II	Hash Table, Math, Dynamic Programming +1
0274	H-Index	Array, Counting Sort, Sorting
0275	H-Index II	Array, Binary Search
0279	Perfect Squares	Breadth-First Search, Math, Dynamic Programming
0284	Peeking Iterator	Design, Array, Iterator
0287	Find the Duplicate Number	Bit Manipulation, Array, Two Pointers +1
0289	Game of Life	Array, Matrix, Simulation
0299	Bulls and Cows	Hash Table, String, Counting
0300	Longest Increasing Subsequence	Array, Binary Search, Dynamic Programming
0304	Range Sum Query 2D - Immutable	Design, Array, Matrix +1
0306	Additive Number	String, Backtracking
0307	Range Sum Query - Mutable	Design, Binary Indexed Tree, Segment Tree +2
0309	Best Time to Buy and Sell Stock with Cooldown	Array, Dynamic Programming
0310	Minimum Height Trees	Depth-First Search, Breadth-First Search, Graph +1
0313	Super Ugly Number	Array, Math, Dynamic Programming
0316	Remove Duplicate Letters	Stack, Greedy, String +1
0318	Maximum Product of Word Lengths	Bit Manipulation, Array, String
0319	Bulb Switcher	Brainteaser, Math
0322	Coin Change	Breadth-First Search, Array, Dynamic Programming
0324	Wiggle Sort II	Greedy, Array, Divide and Conquer +2
0328	Odd Even Linked List	Linked List
0331	Verify Preorder Serialization of a Binary Tree	Stack, Tree, String +1
0334	Increasing Triplet Subsequence	Greedy, Array
0337	House Robber III	Tree, Depth-First Search, Dynamic Programming +1
0341	Flatten Nested List Iterator	Stack, Tree, Depth-First Search +3
0343	Integer Break	Math, Dynamic Programming
0347	Top K Frequent Elements	Array, Hash Table, Divide and Conquer +5
0355	Design Twitter	Design, Hash Table, Linked List +1
0357	Count Numbers with Unique Digits	Math, Dynamic Programming, Backtracking
0365	Water and Jug Problem	Depth-First Search, Breadth-First Search, Math
0368	Largest Divisible Subset	Array, Math, Dynamic Programming +1
0371	Sum of Two Integers	Bit Manipulation, Math
0372	Super Pow	Math, Divide and Conquer
0373	Find K Pairs with Smallest Sums	Array, Heap (Priority Queue)
0375	Guess Number Higher or Lower II	Math, Dynamic Programming, Game Theory
0376	Wiggle Subsequence	Greedy, Array, Dynamic Programming
0377	Combination Sum IV	Array, Dynamic Programming
0378	Kth Smallest Element in a Sorted Matrix	Array, Binary Search, Matrix +2
0380	Insert Delete GetRandom O(1)	Design, Array, Hash Table +2
0382	Linked List Random Node	Reservoir Sampling, Linked List, Math +1
0384	Shuffle an Array	Design, Array, Math +1
0385	Mini Parser	Stack, Depth-First Search, String
0386	Lexicographical Numbers	Depth-First Search, Trie
0388	Longest Absolute File Path	Stack, Depth-First Search, String
0390	Elimination Game	Recursion, Math
0393	UTF-8 Validation	Bit Manipulation, Array
0394	Decode String	Stack, Recursion, String
0395	Longest Substring with At Least K Repeating Characters	Hash Table, String, Divide and Conquer +1
0396	Rotate Function	Array, Math, Dynamic Programming
0397	Integer Replacement	Greedy, Bit Manipulation, Memoization +1
0398	Random Pick Index	Reservoir Sampling, Hash Table, Math +1
0399	Evaluate Division	Depth-First Search, Breadth-First Search, Union Find +4
0400	Nth Digit	Math, Binary Search
0402	Remove K Digits	Stack, Greedy, String +1
0406	Queue Reconstruction by Height	Binary Indexed Tree, Segment Tree, Array +1
0413	Arithmetic Slices	Array, Dynamic Programming, Sliding Window
0416	Partition Equal Subset Sum	Array, Dynamic Programming
0417	Pacific Atlantic Water Flow	Depth-First Search, Breadth-First Search, Array +1
0419	Battleships in a Board	Depth-First Search, Array, Matrix
0421	Maximum XOR of Two Numbers in an Array	Bit Manipulation, Trie, Array +1
0423	Reconstruct Original Digits from English	Hash Table, Math, String
0424	Longest Repeating Character Replacement	Hash Table, String, Sliding Window
0427	Construct Quad Tree	Tree, Array, Divide and Conquer +1
0429	N-ary Tree Level Order Traversal	Tree, Breadth-First Search
0430	Flatten a Multilevel Doubly Linked List	Depth-First Search, Linked List, Doubly-Linked List

The patterns behind medium problems

A medium problem, live, with someone watching.

Approach

Approach

Approach

Medium problems

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