LeetCode Problems

These are my solutions for some of the LeetCode problems. I'm aiming to do at least one a day as part of the daily LeetCoding challenge. Most will be done in C++ or Java since I wanted to brush up on my low-level languages.

You can find me on LeetCode as BillyLjm.

Problems

Num	Title	Solutions	Concepts
5	Longest Palindromic Substring	Manacher's Algorithm	Prefix Suffix
17	Letter Combinations of a Phone Number	Recursion	Combinatorics
20	Valid Parentheses	Stack	Stack
23	Merge k Sorted Lists	Priority Queue	Priority Queue, Pointers
24	Swap Nodes in Pairs	Pointers	Linked Lists
28	Find the Index of the First Occurrence in a String	KMP Substring	Find Patterns
33	Search in Rotated Sorted Array	Modified Binary Search	Search
34	Find First and Last Position of Element in Sorted Array	Binary Search	Search
38	Count and Say	Iteration	Implementation
46	Permutations	In-Built Function	Combinatorics
54	Spiral Matrix	Indexing	Arrays
59	Spiral Matrix II	Robot	Arrays
62	Unique Paths	Dynamic Programming	Maze Searching
63	Unique Paths II	Dynamic Programming	Maze Searching
64	Minimum Path Sum	Dynamic Programming	Path Searching
68	Text Justification	Iteration	Strings
71	Simplify Path	Vector	String Parsing
72	Edit Distance	> Dynamic Programming > Memoisation	Recursion, Memoisation, Dynamic Programming
74	Search a 2D Matrix	Binary Search	Searching
75	Sort Colors	Counting Sort	Sorting
77	Combinations	Recursion	Combinatorics
81	Search in Rotated Sorted Array II	Binary Search	Search
86	Partition List	Pointers	Linked Lists
87	Scramble String	Dynamic Programming	Substrings
92	Reverse Linked List II	Pointers	Linked Lists
95	Unique Binary Search Trees II	Dynamic Programming	Combinatorics
97	Interleaving String	Memoisation	Recursion
101	Symmetric Tree	Tree Traversal	Tree Traversal
106	Construct Binary Tree from Inorder and Postorder Traversal	Recursion, Map	Tree Traversal
109	Convert Sorted List to Binary Search Tree	> Vector > No Mid-Finding	Linked Lists, Binary Trees
118	Pascal's Triangle	Iteration	Iteration
119	Pascal's Triangle II	Dynamic Programming	Iteration
122	Best Time to Buy and Sell Stock II	Greedy	Problem Solving
127	Word Ladder	Bidirectional BFS	Graph Traversal
129	Sum Root to Leaf Numbers	Recursion	Binary Trees
133	Clone Graph	Adjacency List	Undirected Graphs
135	Candy	Two Pass	Problem Solving
138	Copy List with Random Pointer	Maps	Linked Lists
139	Word Break	Memoisation	Recursion
141	Linked List Cycle	Floyd's Cycle Finding	Linked Lists
142	Linked List Cycle II	Floyd's Cycle Finding	Linked Lists
146	LRU Cache	Hashed Doubly-Linked Lists	Data Structures
168	Excel Sheet Column Title	Binary Conversion	Bases
175	Combine Two Tables	Left Join	Joins
176	Second Highest Salary	Max	Max
185	Department Top Three Salaries	Dense Rank	Dense Rank
208	Implement Trie (Prefix Tree)	Map	Trie/Prefix Tree
211	Design Add and Search Words Data Structure	Trie/Prefix Tree	Data Structures
215	Kth Largest Element in an Array	QuickSelect	Sorting
225	Implement Stack using Queues	Double-Ended Queue	Data Structures
228	Summary Ranges	Iteration	Strings
229	Majority Element II	Boyer-Moore Majority Vote	Majority
238	Remove Colored Pieces if Both Neighbors are the Same Color	Greedy	Games
239	Sliding Window Maximum	Double Ended Queue	Sliding Windows
258	Add Digits	Maths	Maths
262	Trips and Users	GroupBy	GroupBy
316	Remove Duplicate Letters	Greedy	Data Structures
319	Bulb Switcher	Maths	Maths
330	Patching Array	Greedy	Integer Ranges
338	Counting Bits	Recursion	Binary Numbers
341	Flatten Nested List Iterator	Stack	Nested Lists
342	Power of Four	Maths	Floats
343	Integer Break	Maths	Maths
347	Top K Frequent Elements	Quickselect	Selection
350	Intersection of Two Arrays II	Sets	Sets
373	Find K Pairs with Smallest Sums	Priority Queue	Permutations
377	Combination Sum IV	Dynamic Programming	Data Types
382	Linked List Random Node	Reservoir Sampling	Reservoir Sampling
389	Find the Difference	ASCII	Data Types
399	Evaluate Division	Depth-First Search	Graph Representation
403	Frog Jump	Memoisation	Recursion
429	Construct Quad Tree	Recursion	Recursion, Memory Allocation
443	String Compression	Iteration	Array Looping
458	Poor Pigs	Tensors	Problem Solving
459	Repeated Substring Pattern	Iteration	Substrings
474	Ones and Zeroes	Dynamic Programming	Knapsack
502	IPO	Greedy	Problem Solving
515	Find Largest Value in Each Tree Row	DFS	Binary Trees
518	Coin Change II	Recursion	Recursion
530	Minimum Absolute Difference in BST	Recursion	Tree Traversal
542	01 Matrix	Breadth-First Search	Mazes
547	Number of Provinces	Union-Find	Disjoint Sets
557	Reverse Words in a String III	Swap	Strings
601	Human Traffic of Stadium	Gaps & Islands	Consecutive
605	Can Place Flowers	Iteration	Logic
633	Sum of Square Numbers	Two Pointers	Integers
646	Maximum Length of Pair Chain	Greedy	Problem Solving
649	Dota2 Senate	Greedy	Data Structures
652	Find Duplicate Subtrees	Hashing	Hashing, Binary Trees
685	Redundant Connection II	Union-Find	Trees
703	Kth Largest Element in a Stream	Priority Queue	Heap
704	Binary Search	Binary Search	Array Searching
705	Design HashSet	Vector	Hashing
706	Design HashMap	Vector	Hashing
714	Best Time to Buy and Sell Stock with Transaction Fee	Iteration	Iteration
725	Split Linked List in Parts	Pointers	Linked Lists
744	Find Smallest Letter Greater Than Target	Binary Search	Search
746	Min Cost Climbing Stairs	Dynamic Programming	Dynamic Programming
767	Reorganize String	String Braiding	Braiding
779	K-th Symbol in Grammar	Binary Tree	Recursion
781	Rabbits in Forest	Counter	Groupings
785	Is Graph Bipartite?	Recursion	Graph Traversal
799	Champagne Tower	Physics	Modelling
823	Binary Trees With Factors	Dynamic Programming	Problem Solving
826	Most Profit Assigning Work	Greedy	Allocation
837	New 21 Game	Dynamic Programming	Problem Solving
839	Similar String Groups	Union-Find	Disjoint Sets
844	Backspace String Compare	Pointers	Strings
864	Shortest Path to Get All Keys	Breadth-First Search	Mazes
875	Koko Eating Bananas	Binary Search	Binary Search, Integer Overflow
881	Boats to Save People	Greedy	Problem Solving
896	Monotonic Array	Iteration	Arrays
905	Sort Array By Parity	Swap	Arrays
912	Sort an Array	HeapSort	Array Sorting
920	Number of Music Playlists	Combinatorics	Modular Arithmetic
934	Shortest Bridge	Breadth-First Search	Graphs
945	Minimum Increment to Make Array Unique	Sorting	Problem Solving
956	Tallest Billboard	Dynamic Programming	Problem Simplification
958	Check Completeness of a Binary Tree	Depth-First Search	Binary Trees
983	Minimum Cost For Tickets	Dynamic Programming	Logic
995	Minimum Number of K Consecutive Bit Flips	Bit-Wise	Bits
1020	Number of Enclaves	Flood Fill	Recursion
1027	Longest Arithmetic Subsequence	Hashmap	Time Complexity
1035	Uncrossed Lines	Dynamic Programming	Subsequences
1038	Binary Search Tree to Greater Sum Tree	Recursion	Tree Traversal
1048	Longest String Chain	Dynamic Programming	Substrings
1052	Grumpy Bookstore Owner	Sliding Window	Iteration
1091	Shortest Path in Binary Matrix	Breadth-First Search	Search
1095	Find in Mountain Array	Binary Search	Sorted Arrays
1139	Largest 1-Bordered Square	Pre-Count	Problem Solving
1140	Stone Game II	Memoisation	Problem Solving
1144	Number of Ways of Cutting a Pizza	Dynamic Programming	Problem Solving
1146	Snapshot Array	Changelog	Version Control
1161	Maximum Level Sum of a Binary Tree	Breadth-First Search	Tree Traversal
1187	Make Array Strictly Increasing	Memoisation	Games
1203	Sort Items by Groups Respecting Dependencies	Topological Sort	Ordering
1220	Count Vowels Permutation	Dynamic Programming	Problem Solving
1232	Check If It Is a Straight Line	Slope	Geometry
1248	Count Number of Nice Subarrays	Sliding Window	Iteration
1254	Number of Closed Islands	Flood Fill	Recursion
1269	Number of Ways to Stay in the Same Place After Some Steps	Dynamic Programming	Problem Solving
1282	Group the People Given the Group Size They Belong To	Greedy	Problem Solving
1318	Minimum Flips to Make a OR b Equal to c	Bitwise	Bitwise Operations
1319	Number of Operations to Make Network Connected	> Union-Find >Depth-First Search	Disjoint Sets
1326	Minimum Number of Taps to Open to Water a Garden	Greedy	Caching
1345	Jump Game IV	Breadth-First Search	Path Finding
1351	Count Negative Numbers in a Sorted Matrix	Iteration	Iteration
1356	Sort Integers by The Number of 1 Bits	Bitwise	Bits
1359	Count All Valid Pickup and Delivery Options	Maths	Integer Overflow
1361	Validate Binary Tree Nodes	Union-Find (Modified)	Binary Trees
1376	Time Needed to Inform All Employees	Depth First Search	Tree Traversal
1382	Balance a Binary Search Tree	Inorder	Binary Search Trees
1396	Design Underground System	Unordered Maps	Data Structures
1402	Reducing Dishes	Greedy	Problem-Solving
1406	Stone Game III	Memoisation	Problem-Solving
1420	Build Array Where You Can Find The Maximum Exactly K Comparisons	Dynamic Programming	Problem-Solving
1425	Constrained Subsequence Sum	Dynamic Programming	Priority Queue
1438	Longest Continuous Subarray With Absolute Diff Less Than or Equal to Limit	Sliding Window	Min/Max in Window
1456	Maximum Number of Vowels in a Substring of Given Length	Sliding Window	Sliding Window
1458	Max Dot Product of Two Subsequences	Dynamic Programming	Problem-Solving
1466	Reorder Routes to Make All Paths Lead to the City Zero	Breadth-First Search	Tree Traversal
1472	Design Browser History	Vector	Data Structures
1482	Minimum Number of Days to Make m Bouquets	Binary Search	Optimisation
1489	Find Critical and Pseudo-Critical Edges in Minimum Spanning Tree	Kruskal's Algorithm	Graphs
1491	Average Salary Excluding the Minimum and Maximum Salary	Iteration	Data Types
1498	Number of Subsequences That Satisfy the Given Sum Condition	Permutation, Modular Exp	Permutation, Integer Overflow
1502	Can Make Arithmetic Progression From Sequence	Iteration	Iteration
1509	Minimum Difference Between Largest and Smallest Value in Three Moves	Sorting	Iteration
1512	Number of Good Pairs	Count	Arrays
1513	Number of Substrings With Only 1s	Combinatorics	Combinatorics
1514	Path with Maximum Probability	Dijkstra's Algorithm	Graph Traversal
1518	Water Bottles	Iteration	Iteration
1526	Minimum Number of Increments on Subarrays to Form a Target Array	Numpy	Problem Solving
1534	Count Good Triplets	Frequency	Code Optimisation
1539	Kth Missing Positive Number	Iteration	Iteration
1547	Minimum Cost to Cut a Stick	Dynamic Programming	Problem Solving
1550	Three Consecutive Odds	Iteration	Iteration
1552	Magnetic Force Between Two Balls	Binary Search	Problem Solving
1557	Minimum Number of Vertices to Reach All Nodes	Topological Sort	Directed Graphs
1569	Number of Ways to Reorder Array to Get Same BST	Recursion	Permutations
1572	Matrix Diagonal Sum	Indexing	Arrays
1575	Count All Possible Routes	Memoisation	Recursion
1578	Minimum Time to Make Rope Colorful	Pointers	Pointers
1579	Remove Max Number of Edges to Keep Graph Fully Traversable	Union-Find	Disjoint Sets
1603	Design Parking System	Vector	Classes
1611	Minimum One Bit Operations to Make Integers Zero	Bitstring	Patterns
1615	Maximal Network Rank	In-Degree	Graphs
1647	Minimum Deletions to Make Character Frequencies Unique	Iteration	Strings
1697	Checking Existence of Edge Length Limited Paths	Union-Find	Disjoint Sets
1721	Swapping Nodes in a Linked List	Value Swap	Linked Lists
1732	Find the Highest Altitude	Iteration	Arrays
1791	Find Center of Star Graph	Direct Answer	-
1793	Maximum Score of a Good Subarray	Greedy	Problem Solving
1799	Maximize Score After N Operations	Dynamic Programming	Substructure, Data Types
1802	Maximum Value at a Given Index in a Bounded Array	Binary Search	Problem Solving
1822	Sign of the Product of an Array	Iteration	Minimal Resource
1857	Largest Color Value in a Directed Graph	Topological Sort	Directed Graphs
1922	Count Good Numbers	Permutations	Permutations
1964	Find the Longest Valid Obstacle Course at Each Position	Dynamic Programming	Permutations
1970	Last Day Where You Can Still Cross	Union-Find	Disjoint Sets
2009	Minimum Number of Operations to Make Array Continuous	Sliding Window	Problem Solving
2037	Minimum Number of Moves to Seat Everyone	Direct Solution	Problem Solving
2050	Parallel Courses III	Topological Sort	Directed Graphs
2058	Find the Minimum and Maximum Number of Nodes Between Critical Points	Crawl	Linked Lists
2090	K Radius Subarray Averages	Sliding Window	Iteration
2101	Detonate the Maximum Bombs	Sweepline DFS	Geometry
2125	Number of Laser Beams in a Bank	Iteration	Problem Formulation
2130	Maximum Twin Sum of a Linked List	Reverse Linked List	Linked Lists
2140	Solving Questions With Brainpower	Dynamic Programming	Optimal Substructure
2145	Count the Hidden Sequences	Intervals	Sets
2169	Count Operations to Obtain Zero	Remainders	Real Numbers
2176	Count Equal and Divisible Pairs in an Array	Dictionary	Iteration
2181	Merge Nodes in Between Zeros	Pointers	Linked Lists
2187	Minimum Time to Complete Tips	Binary Search	Binary Search, Integer Overflow
2215	Find the Difference of Two Arrays	Hashmap	Arrays
2251	Number of Flowers in Full Bloom	Binary Search	Ranges
2257	Count Unguarded Cells in the Grid	Iteration	Simulation
2285	Maximum Total Importance of Roads	Greedy	Graphs
2300	Successful Pairs of Spells and Potions	Array Sorting	Sorted Arrays
2316	Count Unreachable Pairs of Nodes in an Undirected Graph	Union-Find	Disjoint Sets
2328	Number of Increasing Paths in a Grid	Dynamic Programming	Problem Solving
2336	Smallest Number in Infinite Set	Sets	Ordered Sets
2348	Number of Zero-Filled Subarrays	Iteration	Permutations
2352	Equal Row and Column Pairs	Hashmap	Value Lookup
2356	Increment Submatrices by One	2D Difference Array	Matrix
2360	Longest Cycle in a Graph	Topological Sort	Graph Cycles
2366	Minimum Replacements to Sort the Array	Iteration	Problem Solving
2369	Check if There is a Valid Partition For The Array	Dynamic Programming	Pattern Finding
2390	Removing Stars From a String	Stack	Iteration
2405	Optimal Partition of String	Greedy	Problem Solving
2433	Find The Original Array of Prefix XOR	Maths	Bitwise Operations
2439	Minimize Maximum of Array	Greedy	Problem Solving
2444	Count Subarrays w/ Fixed Bounds	Iteration	Iteration
2448	Minimum Cost to Make Array Equal	Convex Optimisation	Optimisation
2462	Total Cost to Hire K Workers	Priority Queue	Queues
2466	Count Ways To Build Good Strings	Dynamic Programming	Optimal Substructure
2483	Minimum Penalty for a Shop	Iteration	Iteration
2492	Minimum Score of a Path Between Two Cities	Union-Find	Disjoint Sets
2528	Maximize the Minimum Powered City	Binary Search + Greedy	Problem Solving
2537	Count the Number of Good Subarrays	Sliding Window	Problem Solving
2542	Maximum Subsequence Score	Problem Reduction	Problem Solving
2563	Count the Number of Fair Pairs	Binary Bisection	Binary Search
2582	Pass the Pillow	Maths	Problem Solving
2616	Minimize the Maximum Difference of Pairs	Binary Search, Greedy	Problem Solving
2654	Minimum Number of Operations to Make All Array Elements Equal to 1	Iteration	Puzzles
2707	Extra Characters in a String	Memoisation	Substrings
2742	Painting the Walls	Dynamic Programming	Problem Solving
3217	Delete Nodes From Linked List Present in Array	Linked List	Linked List
3228	Maximum Number of Operations to Move Ones to the End	Iteration	Puzzles
3234	Count the Number of Substrings With Dominant Ones	Short-Circuit	Puzzles
3272	Find the Count of Good Integers	Permutations	Permutations
3318	Find X-Sum of All K-Long Subarrays I	Rolling Window, Priority Queues	Lists
3354	Make Array Elements Equal to Zero	Rolling Sum	Problem Solving
3370	Smallest Number With All Set Bits	Logarithm	Maths
3542	Minimum Operations to Convert All Elements to Zero	Monotonic Stack	Real Numbers
3607	Power Grid Maintenance	Union-Find & Minheap	Disjoint Sets