Hard LeetCode Problems: All 661,
With Python Solutions

Hard problems usually combine two techniques, or need a non-obvious invariant before any technique applies at all. They are worth studying for the ideas rather than for the odds of being asked one.

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The patterns behind hard problems

The six patterns with the most Hard-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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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...

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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...

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Primary display

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

Page 1 of 4

Hard problems

Showing the first 200 of 661 problems. Problems with a complete Python solution are listed first, then by ascending problem number.

Hard problems
No.	Problem	Topics
0004	Median of Two Sorted Arrays	Array, Binary Search, Divide and Conquer
0010	Regular Expression Matching	Recursion, String, Dynamic Programming
0023	Merge k Sorted Lists	Linked List, Divide and Conquer, Heap (Priority Queue) +1
0025	Reverse Nodes in k-Group	Recursion, Linked List
0030	Substring with Concatenation of All Words	Hash Table, String, Sliding Window
0032	Longest Valid Parentheses	Stack, String, Dynamic Programming
0037	Sudoku Solver	Array, Hash Table, Backtracking +1
0041	First Missing Positive	Array, Hash Table
0042	Trapping Rain Water	Stack, Array, Two Pointers +2
0044	Wildcard Matching	Greedy, Recursion, String +1
0051	N-Queens	Array, Backtracking
0052	N-Queens II	Backtracking
0060	Permutation Sequence	Recursion, Math
0065	Valid Number	String
0068	Text Justification	Array, String, Simulation
0076	Minimum Window Substring	Hash Table, String, Sliding Window
0084	Largest Rectangle in Histogram	Stack, Array, Monotonic Stack
0085	Maximal Rectangle	Stack, Array, Dynamic Programming +2
0087	Scramble String	String, Dynamic Programming
0115	Distinct Subsequences	String, Dynamic Programming
0123	Best Time to Buy and Sell Stock III	Array, Dynamic Programming
0124	Binary Tree Maximum Path Sum	Tree, Depth-First Search, Dynamic Programming +1
0126	Word Ladder II	Breadth-First Search, Hash Table, String +1
0127	Word Ladder	Breadth-First Search, Hash Table, String
0132	Palindrome Partitioning II	String, Dynamic Programming
0135	Candy	Greedy, Array
0140	Word Break II	Trie, Memoization, Array +4
0149	Max Points on a Line	Geometry, Array, Hash Table +1
0154	Find Minimum in Rotated Sorted Array II	Array, Binary Search
0174	Dungeon Game	Array, Dynamic Programming, Matrix
0185	Department Top Three Salaries	Database
0188	Best Time to Buy and Sell Stock IV	Array, Dynamic Programming
0212	Word Search II	Trie, Array, String +2
0214	Shortest Palindrome	String, String Matching, Hash Function +1
0218	The Skyline Problem	Binary Indexed Tree, Segment Tree, Array +5
0220	Contains Duplicate III	Array, Bucket Sort, Ordered Set +2
0224	Basic Calculator	Stack, Recursion, Math +1
0233	Number of Digit One	Recursion, Math, Dynamic Programming
0239	Sliding Window Maximum	Queue, Array, Sliding Window +2
0262	Trips and Users	Database
0273	Integer to English Words	Recursion, Math, String
0282	Expression Add Operators	Math, String, Backtracking
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
0301	Remove Invalid Parentheses	Breadth-First Search, String, Backtracking
0312	Burst Balloons	Array, Dynamic Programming
0315	Count of Smaller Numbers After Self	Binary Indexed Tree, Segment Tree, Array +4
0321	Create Maximum Number	Stack, Greedy, Array +2
0327	Count of Range Sum	Binary Indexed Tree, Segment Tree, Array +4
0329	Longest Increasing Path in a Matrix	Depth-First Search, Breadth-First Search, Graph +5
0330	Patching Array	Greedy, Array
0332	Reconstruct Itinerary	Depth-First Search, Graph, Eulerian Circuit
0335	Self Crossing	Geometry, Array, Math
0336	Palindrome Pairs	Trie, Array, Hash Table +1
0352	Data Stream as Disjoint Intervals	Union Find, Design, Hash Table +3
0354	Russian Doll Envelopes	Array, Binary Search, Dynamic Programming +1
0363	Max Sum of Rectangle No Larger Than K	Array, Binary Search, Matrix +2
0381	Insert Delete GetRandom O(1) - Duplicates allowed	Design, Array, Hash Table +2
0391	Perfect Rectangle	Geometry, Array, Hash Table +2
0403	Frog Jump	Array, Dynamic Programming
0407	Trapping Rain Water II	Breadth-First Search, Array, Matrix +1
0410	Split Array Largest Sum	Greedy, Array, Binary Search +2
0420	Strong Password Checker	Greedy, String, Heap (Priority Queue)
0432	All O`one Data Structure	Design, Hash Table, Linked List +1
0440	K-th Smallest in Lexicographical Order	Trie
0446	Arithmetic Slices II - Subsequence	Array, Dynamic Programming
0458	Poor Pigs	Math, Dynamic Programming, Combinatorics
0460	LFU Cache	Design, Hash Table, Linked List +1
0466	Count The Repetitions	String, Dynamic Programming
0472	Concatenated Words	Depth-First Search, Trie, Array +3
0479	Largest Palindrome Product	Math, Enumeration
0480	Sliding Window Median	Array, Hash Table, Sliding Window +1
0483	Smallest Good Base	Math, Binary Search
0488	Zuma Game	Stack, Breadth-First Search, Memoization +2
0493	Reverse Pairs	Binary Indexed Tree, Segment Tree, Array +4
0502	IPO	Greedy, Array, Sorting +1
0514	Freedom Trail	Depth-First Search, Breadth-First Search, String +1
0517	Super Washing Machines	Greedy, Array
0546	Remove Boxes	Memoization, Array, Dynamic Programming
0552	Student Attendance Record II	Dynamic Programming
0564	Find the Closest Palindrome	Math, String
0587	Erect the Fence	Geometry, Array, Math
0591	Tag Validator	Stack, String
0600	Non-negative Integers without Consecutive Ones	Dynamic Programming
0601	Human Traffic of Stadium	Database
0629	K Inverse Pairs Array	Dynamic Programming
0630	Course Schedule III	Greedy, Array, Sorting +1
0632	Smallest Range Covering Elements from K Lists	Greedy, Array, Hash Table +3
0639	Decode Ways II	String, Dynamic Programming
0664	Strange Printer	String, Dynamic Programming
0668	Kth Smallest Number in Multiplication Table	Math, Binary Search
0675	Cut Off Trees for Golf Event	Breadth-First Search, Array, Matrix +1
0679	24 Game	Array, Math, Backtracking
0685	Redundant Connection II	Depth-First Search, Breadth-First Search, Union Find +1
0689	Maximum Sum of 3 Non-Overlapping Subarrays	Array, Dynamic Programming, Prefix Sum +1
0691	Stickers to Spell Word	Bit Manipulation, Memoization, Array +5
0699	Falling Squares	Segment Tree, Array, Ordered Set
0710	Random Pick with Blacklist	Array, Hash Table, Math +3
0715	Range Module	Design, Segment Tree, Ordered Set
0719	Find K-th Smallest Pair Distance	Array, Two Pointers, Binary Search +1
0730	Count Different Palindromic Subsequences	String, Dynamic Programming
0732	My Calendar III	Design, Segment Tree, Binary Search +2
0736	Parse Lisp Expression	Stack, Recursion, Hash Table +1
0741	Cherry Pickup	Array, Dynamic Programming, Matrix
0745	Prefix and Suffix Search	Design, Trie, Array +2
0749	Contain Virus	Depth-First Search, Breadth-First Search, Array +2
0753	Cracking the Safe	Depth-First Search, Graph, Eulerian Circuit
0757	Set Intersection Size At Least Two	Greedy, Array, Sorting
0761	Special Binary String	Recursion, String
0765	Couples Holding Hands	Greedy, Depth-First Search, Breadth-First Search +2
0768	Max Chunks To Make Sorted II	Stack, Greedy, Array +2
0773	Sliding Puzzle	Breadth-First Search, Memoization, Array +3
0778	Swim in Rising Water	Depth-First Search, Breadth-First Search, Union Find +4
0780	Reaching Points	Math
0782	Transform to Chessboard	Bit Manipulation, Array, Math +1
0793	Preimage Size of Factorial Zeroes Function	Math, Binary Search
0798	Smallest Rotation with Highest Score	Array, Prefix Sum
0801	Minimum Swaps To Make Sequences Increasing	Array, Dynamic Programming
0803	Bricks Falling When Hit	Union Find, Array, Matrix
0805	Split Array With Same Average	Bit Manipulation, Array, Math +2
0810	Chalkboard XOR Game	Bit Manipulation, Brainteaser, Array +2
0815	Bus Routes	Breadth-First Search, Array, Hash Table
0818	Race Car	Dynamic Programming
0827	Making A Large Island	Depth-First Search, Breadth-First Search, Union Find +2
0828	Count Unique Characters of All Substrings of a Given String	Hash Table, String, Dynamic Programming
0829	Consecutive Numbers Sum	Math, Enumeration
0834	Sum of Distances in Tree	Tree, Depth-First Search, Graph +1
0839	Similar String Groups	Depth-First Search, Breadth-First Search, Union Find +3
0847	Shortest Path Visiting All Nodes	Bit Manipulation, Breadth-First Search, Graph +2
0850	Rectangle Area II	Segment Tree, Array, Ordered Set +1
0854	K-Similar Strings	Breadth-First Search, Hash Table, String
0857	Minimum Cost to Hire K Workers	Greedy, Array, Sorting +1
0862	Shortest Subarray with Sum at Least K	Queue, Array, Binary Search +4
0864	Shortest Path to Get All Keys	Bit Manipulation, Breadth-First Search, Array +1
0871	Minimum Number of Refueling Stops	Greedy, Array, Dynamic Programming +1
0878	Nth Magical Number	Math, Binary Search
0879	Profitable Schemes	Array, Dynamic Programming
0882	Reachable Nodes In Subdivided Graph	Graph, Shortest Path, Heap (Priority Queue)
0887	Super Egg Drop	Math, Binary Search, Dynamic Programming
0891	Sum of Subsequence Widths	Array, Math, Sorting
0895	Maximum Frequency Stack	Stack, Design, Hash Table +1
0899	Orderly Queue	Math, String, Sorting
0902	Numbers At Most N Given Digit Set	Array, Math, String +2
0903	Valid Permutations for DI Sequence	String, Dynamic Programming, Prefix Sum
0906	Super Palindromes	Math, String, Enumeration
0913	Cat and Mouse	Graph, Topological Sort, Memoization +3
0920	Number of Music Playlists	Math, Dynamic Programming, Combinatorics
0924	Minimize Malware Spread	Depth-First Search, Breadth-First Search, Union Find +3
0927	Three Equal Parts	Array, Math
0928	Minimize Malware Spread II	Depth-First Search, Breadth-First Search, Union Find +3
0936	Stamping The Sequence	Stack, Greedy, Queue +1
0940	Distinct Subsequences II	String, Dynamic Programming
0943	Find the Shortest Superstring	Bit Manipulation, Array, String +2
0952	Largest Component Size by Common Factor	Union Find, Array, Hash Table +2
0956	Tallest Billboard	Array, Dynamic Programming
0960	Delete Columns to Make Sorted III	Array, String, Dynamic Programming
0964	Least Operators to Express Number	Memoization, Math, Dynamic Programming
0968	Binary Tree Cameras	Tree, Depth-First Search, Dynamic Programming +1
0975	Odd Even Jump	Stack, Array, Dynamic Programming +3
0980	Unique Paths III	Bit Manipulation, Array, Backtracking +1
0982	Triples with Bitwise AND Equal To Zero	Bit Manipulation, Array, Hash Table
0987	Vertical Order Traversal of a Binary Tree	Tree, Depth-First Search, Breadth-First Search +3
0992	Subarrays with K Different Integers	Array, Hash Table, Counting +1
0995	Minimum Number of K Consecutive Bit Flips	Bit Manipulation, Queue, Array +2
0996	Number of Squareful Arrays	Bit Manipulation, Array, Hash Table +4
1000	Minimum Cost to Merge Stones	Array, Dynamic Programming, Prefix Sum
1001	Grid Illumination	Array, Hash Table
1012	Numbers With Repeated Digits	Math, Dynamic Programming
1028	Recover a Tree From Preorder Traversal	Tree, Depth-First Search, String +1
1032	Stream of Characters	Design, Trie, Array +2
1036	Escape a Large Maze	Depth-First Search, Breadth-First Search, Array +1
1044	Longest Duplicate Substring	String, Binary Search, Suffix Array +3
1074	Number of Submatrices That Sum to Target	Array, Hash Table, Matrix +1
1092	Shortest Common Supersequence	String, Dynamic Programming
1095	Find in Mountain Array	Array, Binary Search, Interactive
1096	Brace Expansion II	Stack, Breadth-First Search, Hash Table +3
1106	Parsing A Boolean Expression	Stack, Recursion, String
1125	Smallest Sufficient Team	Bit Manipulation, Array, Dynamic Programming +1
1147	Longest Chunked Palindrome Decomposition	Greedy, Two Pointers, String +3
1157	Online Majority Element In Subarray	Design, Binary Indexed Tree, Segment Tree +2
1163	Last Substring in Lexicographical Order	Two Pointers, String
1172	Dinner Plate Stacks	Stack, Design, Hash Table +1
1178	Number of Valid Words for Each Puzzle	Bit Manipulation, Trie, Array +2
1187	Make Array Strictly Increasing	Array, Binary Search, Dynamic Programming +1
1192	Critical Connections in a Network	Depth-First Search, Graph, Biconnected Component
1203	Sort Items by Groups Respecting Dependencies	Depth-First Search, Breadth-First Search, Graph +1
1206	Design Skiplist	Design, Linked List
1210	Minimum Moves to Reach Target with Rotations	Breadth-First Search, Array, Matrix
1220	Count Vowels Permutation	Dynamic Programming
1223	Dice Roll Simulation	Array, Dynamic Programming
1224	Maximum Equal Frequency	Array, Hash Table
1235	Maximum Profit in Job Scheduling	Array, Binary Search, Dynamic Programming +1
1240	Tiling a Rectangle with the Fewest Squares	Backtracking
1250	Check If It Is a Good Array	Array, Math, Number Theory
1255	Maximum Score Words Formed by Letters	Bit Manipulation, Array, Hash Table +5
1263	Minimum Moves to Move a Box to Their Target Location	Breadth-First Search, Array, Matrix +1
1269	Number of Ways to Stay in the Same Place After Some Steps	Dynamic Programming
1278	Palindrome Partitioning III	String, Dynamic Programming
1284	Minimum Number of Flips to Convert Binary Matrix to Zero Matrix	Bit Manipulation, Breadth-First Search, Array +2
1289	Minimum Falling Path Sum II	Array, Dynamic Programming, Matrix

The patterns behind hard problems

A hard problem, live, with someone watching.

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Hard problems

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