Leetcode and Java

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Description

The project is divided into two parts: structure and solution.

structure package contains the data structure required for the question, such as TreeNode for the binary tree question, ListNode for the linked list question, etc.

solution package contains the leetcode algorithm problems, packaged by solution type.

Problems

Data Structure

No.	Title	Short Description
1	TreeNode	Definition for the Binary Tree node. - construct List by Integer Array. - print the Binary Tree with level order.
2	ListNode	Definition for the LinkedList node. - construct List by Integer Array. - construct by value. - set cycle(to solve the cycle problems). - print the LinkedList.
3	Node	Definition for the Tree node with next right pointers. - construct List by Integer Array.
4	NNode	Definition for the N-ary Tree node. - construct List by Integer Array. - construct by value.

Utils

Tool class for handling issues such as format conversions.

No.	Title	Short Description
1	InputUtils.java	Handle Input Data Format -java int[][] generateGrid(int m, int n, String s) : convert String like "[[0,1],[0,2]]" to grid.

Classification

LinkedList

Reverse

No.	Title	Difficulty	Solution	Idea
92	Reverse Linked List II	Medium	ReverseLinkedListII.java	Iteration | Recursion Reverse Linked List between a and b. At first, we can move to a-th, and the problem becomes Reverse Top N nodes.
25	Reverse Nodes in k-Group	Hard	ReverseNodesInkGroup.java	For each k group, reverse Linked List between head and k-th. Let head equal k+1th node, then do Recursion.
234	Palindrome Linked List	Easy	PalindromeLinkedList.java	1. Using a pointer that points to the node from the start location. Recursion and move pointer forward. Compare. 2. Reverse the whole List. 3. Reverse the second-half List
143	Reorder List	Easy	ReorderList.java	Reverse the second-half List and Merge
147	Insertion Sort List	Easy	InsertionSortList.java	Virtual Head Node
206	Reverse Linked List	Easy	ReverseLinkedList.java	Iteration | Recursion
2130	Maximum Twin Sum of a Linked List	Medium	MaximumTwinSumOfALinkedList.java	Reverse the second-half List

Two Pointer

No.	Title	Difficulty	Solution	Idea
160	Intersection of Two Linked Lists	Easy	IntersectionOfTwoLinkedLists.java	Hash | A + B

Design

No.	Title	Difficulty	Solution	Idea
341	Flatten Nested List Iterator	Medium	FlattenNestedListIterator.java	LinkedList, Deque, or Stack.
348	Design Tic-Tac-Toe	Medium	DesignTicTacToe.java	using Array.
380	Insert Delete GetRandom O(1)	Medium	InsertDeleteGetRandom.java	HashMap + ArrayList
622	Design Circular Queue	Medium	DesignCircularQueue.java	Using Array and Pointer.
705	Design HashSet	Easy	DesignHashSet.java	Array + LinkedList
706	Design HashMap	Easy	DesignHashMap.java	Array + LinkedList
1166	Design File System	Medium	DesignFileSystem.java	- 1. Trie: using HashMap as the child list.
1396	Design Underground System	Medium	DesignUndergroundSystem.java	using two HashMap. One saves the check-in record, another saves the data of the current stage.
2102	Sequentially Ordinal Rank Tracker	Hard	SequentiallyOrdinalRankTracker.java	Two Heap.

Tree

Binary Tree

Traversal

No.	Title	Difficulty	Solution	Idea
94	Binary Tree Inorder Traversal	Easy	BinaryTreeInorderTraversal.java	Recursion and Iteration
144	Binary Tree Preorder Traversal	Easy	BinaryTreePreorderTraversal.java	Recursion and Iteration
145	Binary Tree Postorder Traversal	Easy	BinaryTreePostorderTraversal.java	Recursion and Iteration
314	Binary Tree Vertical Order Traversal	Medium	BinaryTreeVerticalOrderTraversal.java	Recursion and Iteration
102	Binary Tree Level Order Traversal	Medium	BinaryTreeLevelOrderTraversal.java	BFS

Construct

No.	Title	Difficulty	Solution	Idea

Serialize and Deserialize

No.	Title	Difficulty	Solution	Idea

No.	Title	Difficulty	Solution	Idea
108	Convert Sorted Array to Binary Search Tree	Medium	ConvertSortedArrayToBinarySearchTree.java	DFS
129	Sum Root to Leaf Numbers	Medium	SumRootOoLeafNumbers.java	DFS
199	Binary Tree Right Side View	Medium	BinaryTreeRightSideView.java	DFS
366	Find Leaves of Binary Tree	Medium	FindLeavesOfBinaryTree.java	Postorder traversal
404	Sum of Left Leaves	Easy	SumOfLeftLeaves.java	DFS
513	Find Bottom Left Tree Value	Medium	FindBottomLeftTreeValue.java	DFS
515	Find Largest Value in Each Tree Row	Medium	FindLargestValueInEachTreeRow.java	BFS |DFS
617	Merge Two Binary Trees	Easy	MergeTwoBinaryTrees.java	DFS
662	Maximum Width of Binary Tree	Medium	MaximumWidthOfBinaryTree.java	BFS |DFS + The relationship of the index between the father node and the child node.
979	Distribute Coins in Binary Tree	Medium	DistributeCoinsInBinaryTree.java	DFS
1302	Deepest Leaves Sum	Medium	DeepestLeavesSum.java	BFS | DFS
1379	Find a Corresponding Node of a Binary Tree in a Clone of That Tree	Medium	FindACorrespondingNodeOfABinaryTreeInACloneOfThatTree.java	inorder traversal
2265	Count Nodes Equal to Average of Subtree	Medium	CountNodesEqualToAverageOfSubtree.java	Postorder traversal

Binary Search Tree

No.	Title	Difficulty	Solution	Idea
173	Binary Search Tree Iterator	Medium	BinarySearchTreeIterator.java
230	Kth Smallest Element in a BST	Medium	KthSmallestElementInABST.java	Preorder traversal

Complete Tree

No.	Title	Difficulty	Solution	Idea
222	Count Complete Tree Nodes	Medium	CountCompleteTreeNodes.java	Consider left and right separatly

Backtrack

No.	Title	Difficulty	Solution	Idea
17	Letter Combinations of a Phone Number	Medium	LetterCombinationsPhoneNumber.java	Backtrack
39	Combination Sum	Medium	CombinationSum.java	Backtrack
40	Combination Sum II	Medium	CombinationSumII.java	Backtrack
46	Permutations	Medium	Permutations.java	Backtrack
47	Permutations II	Medium	PermutationsII.java	Backtrack
51	N-Queens	Hard	NQueens.java	Backtrack + Memo
52	N-Queens II	Hard	NQueensII.java	Backtrack + Memo
77	Combinations	Medium	Combinations.java	Backtrack
78	Subsets	Medium	Subset.java	Backtrack
79	Word Search	Medium	WordSearch.java	Backtrack
90	Subsets II	Medium	SubsetII.java	Backtrack
93	Restore IP Addresses	Medium	RestoreIPAddresses.java	Backtrack
95	Unique Binary Search Trees II	Medium	UniqueBinarySearchTreesII.java	Backtrack
113	Path Sum II	Medium	PathSumII.java	Backtrack
216	Combination Sum III	Medium	CombinationSumIII.java	Backtrack

DFS

Search Path

No.	Title	Difficulty	Solution	Idea
329	Longest Increasing Path in a Matrix	Hard	LongestIncreasingPathInAMatrix.java	DFS + Memo
547	Number of Provinces	Medium	NumberOfProvinces.java	DFS
802	Find Eventual Safe States	Hard	LongestIncreasingPathInAMatrix.java	DFS + Memo
2267	Check if There Is a Valid Parentheses String Path	Medium	FindEventualSafeStates.java	DFS

Island

No.	Title	Difficulty	Solution	Idea
200	Number of Islands	Medium	NumberOfIslands.java	DFS | Remove the island and count
1254	Number of Closed Islands	Medium	NumberOfClosedIslands.java	DFS | Remove the island close to the border, then Remove other islands and count.
695	Max Area of Island	Medium	MaxAreaOfIsland.java	DFS | Remove the island and count the area of each island. Return the max value.
694	Number of Distinct Islands	Medium	NumberOfDistinctIslands.java	DFS | remove the island and record the path, which could reflect the shape of the island, use HashSet to save the distinct path.
1905	Count Sub Islands	Medium	CountSubIslands.java	DFS | At first, remove the island that is not sub island. Then, remove the island and count.
1020	Number of Enclaves	Medium	NumberOfEnclaves.java	DFS | Remove the island close to the border, then Remove other islands and count. Same as problem '1254'.

BFS

No.	Title	Difficulty	Solution	Idea
752	Open the Lock	Medium	OpenTheLock.java	BFS with Queue
752	Network Delay Time	Medium	NetworkDelayTime.java	directed graph BFS | DFS
286	Walls and Gates	Medium	WallAndGates.java	BFS: spread from the gate
994	Rotting Oranges	Medium	RottingOranges.java	BFS: spread from the rotten oranges
1091	Shortest Path in Binary Matrix	Medium	ShortestPathInBinaryMatrix.java	BFS: spread from (0,0). The path that arrived earliest is the shortest.
1197	Minimum Knight Moves	Medium	MinimumKnightMoves.java	BFS: spread from (0,0). The path that arrived earliest is the shortest.

Array

No.	Title	Difficulty	Solution	Idea
31	Next Permutation	Medium	NextPermutation.java	Find the first element that is greater than the previous element, then replace it with the greater and smallest element on the right side.
228	Summary Ranges	Easy	SummaryRanges.java	Two Pointer.
281	Zigzag Iterator	Medium	ZigzagIterator.java	Two Pointer Muti Pointer or Queue for the Follow-Up Question.
386	Lexicographical Numbers	Medium	LexicographicalNumbers.java	Recursion
412	Fizz Buzz	Easy	FizzBuzz.java	For loop. Calculate i%5 and i%3 separately.
456	132 Pattern	Medium	OneThreeTwoPattern.java	Using Array as a Stack
495	Teemo Attacking	Easy	TeemoAttacking.java	Simulation
605	Can Place Flowers	Easy	CanPlaceFlowers.java	One Pass and Check
768	Max Chunks To Make Sorted II	Hard	MaxChunksToMakeSortedII.java	Iterate through the array, each time all elements to the left are smaller (or equal) to all elements to the right, there is a new chunk.
769	Max Chunks To Make Sorted	Medium	MaxChunksToMakeSorted.java	Iterate through the array, each time the maximum value of all elements to the left equals the index, there is a new chunk.
953	Verifying an Alien Dictionary	Easy	VerifyingAnAlienDictionary.java	Compare adjacent elements
2239	Find Closest Number to Zero	Easy	FindClosestNumberToZero.java	One Pass
1480	Running Sum of 1d Array	Easy	RunningSumOf1dArray.java	Prefix sum
2284	Sender With Largest Word Count	Medium	SenderWithLargestWordCount.java	Priority Queue
2303	Calculate Amount Paid in Taxes	Easy	CalculateAmountPaidInTaxes.java	One Pass
x	Rearrange Sorted Array In MaxMin Form	Medium	RearrangeSortedArrayInMaxMinForm.java	Using % and /

Two Pointer

No.	Title	Difficulty	Solution	Idea
88	Merge Sorted Array	Easy	MergeSortedArray.java	Two Pointer
581	Shortest Unsorted Continuous Subarray	Medium	ShortestUnsortedContinuousSubarray.java	Two Pointer | Sort
905	Sort Array By Parity	Easy	SortArrayByParity.java	Two Pointer
1679	Max Number of K-Sum Pairs	Medium	MaxNumberOfKSumPairs.java	Two Pointer
1695	Maximum Erasure Value	Medium	MaximumErasureValue.java	Two Pointer | Prefix Sum

Intervals

No.	Title	Difficulty	Solution	Idea
56	Merge Intervals	Medium	MergeIntervals.java	Sort and compare left border of the interval
57	Insert Interval	Medium	InsertInterval.java	Add Before, Merge, Add After.
986	Interval List Intersections	Medium	IntervalListIntersections.java	Two pointer and Check intersect
1272	Remove Interval	Medium	RemoveInterval.java	Check Intersection
1288	Remove Covered Intervals	Medium	RemoveCoveredIntervals.java	Sort and compare the right border of the interval

n Sum

No.	Title	Difficulty	Solution	Idea
167	Two Sum II - Input Array Is Sorted	Medium	TwoSumIIInputArrayIsSorted.java	Two Pointer.

voting

No.	Title	Difficulty	Solution	Idea
169	Majority Element	Easy	MajorityElement.java	Sort | Boyer-Moore Voting Algorithm
229	Majority Element II	Medium	MajorityElementII.java	Sort | Boyer-Moore Voting Algorithm
1150	Check If a Number Is Majority Element in a Sorted Array	Easy	CheckIfANumberIsMajorityElementInASortedArray.java	Voting

Matrix

No.	Title	Difficulty	Solution	Idea
54	Spiral Matrix	Medium	SpiralMatrix.java	4 Bound
59	Spiral Matrix II	Medium	SpiralMatrixII.java	4 Bound
867	Transpose Matrix	Easy	TransposeMatrix.java	Traversal

Binary Search

No.	Title	Difficulty	Solution	Idea
374	Guess Number Higher or Lower	Easy	GuessNumberHigherOrLower.java	Binary Search

String

No.	Title	Difficulty	Solution	Idea
5	Longest Palindromic Substring	Medium	LongestPalindromicSubstring.java	Two Pointer, Expand Around Possible Centers
205	Isomorphic Strings	Easy	IsomorphicStrings.java	Use HashMap and HashSet.
535	Encode and Decode TinyURL	Medium	EncodeAndDecodeTinyURL.java	HashMap
647	Palindromic Substrings	Medium	PalindromicSubstrings.java	Two Pointer, Expand Around Possible Centers
844	Backspace String Compare	Medium	BackspaceStringCompare.java	Stack | Two Pointer
1209	Remove All Adjacent Duplicates in String II	Medium	RemoveAllAdjacentDuplicatesInStringII.java	Stack | Two Pointer
2027	Minimum Moves to Convert String	Easy	MinimumMovesToConvertString.java	Pointer and Count
2264	Largest 3-Same-Digit Number in String	Easy	LargestThreeSameDigitNumberInString.java	Two Pointer
2288	Apply Discount to Prices	Medium	ApplyDiscountToPrices.java	Split, Format, StringBuilder

Two Pointer

No.	Title	Difficulty	Solution	Idea
1332	Remove Palindromic Subsequences	Easy	RemovePalindromicSubsequences.java	Two Pointer, check if it is a Palindrome String.

Slide Window

No.	Title	Difficulty	Solution	Idea
3	Longest Substring Without Repeating Characters	Medium	LongestSubstringWithoutRepeatingCharacters.java	Slide Window and HashMap
567	Permutation in String	Medium	PermutationInString.java	Slide Window
1423	Maximum Points You Can Obtain from Cards	Medium	MaximumPointsYouCanObtainFromCards.java	Slide Window
1658	Minimum Operations to Reduce X to Zero	Medium	MinimumOperationsToReduceXToZero.java	Slide Window

Hash Table

No.	Title	Difficulty	Solution	Idea
451	Sort Characters By Frequency	Medium	SortCharactersByFrequency.java	Map and Sort. Or Bucket Sort.
567	Longest Consecutive Sequence	Medium	LongestConsecutiveSequence.java	Hash Table: using HashSet save the nums.

Graph

No.	Title	Difficulty	Solution	Idea
261	Graph Valid Tree	Medium	GraphValidTree.java	This question is actually to determine whether there is a cycle in the graph. - generate Adjacency List - DFS - BFS
399	Evaluate Division	Medium	EvaluateDivision.java	- generate Adjacency List - DFS - BFS
841	Keys and Rooms	Medium	KeysAndRooms.java	- DFS - BFS
133	Clone Graph	Medium	CloneGraph.java	- DFS
277	Find the Celebrity	Medium	FindTheCelebrity.java	- Logical Deduction
310	Minimum Height Trees	Medium	MinimumHeightTrees.java	- BFS: remove leaves
1192	Critical Connections in a Network	Hard	CriticalConnectionsInANetwork.java	- DFS: find cycle and remove the edge.
2368	Reachable Nodes With Restrictions	Medium	ReachableNodesWithRestrictions.java	- DFS.

Bipartite Graph

No.	Title	Difficulty	Solution	Idea
785	Is Graph Bipartite?	Medium	IsBipartite.java	- Paint with different color - DFS -BFS
886	Possible Bipartite	Medium	PossibleBipartition.java	- Paint with different color - DFS -BFS

Dynamic Programming

No.	Title	Difficulty	Solution	Idea
63	Unique Paths II	Medium	UniquePathsII.java	DP
120	Triangle	Medium	Triangle.java	DP
256	Paint House	Medium	PaintHouse.java	DP
300	Longest Increasing Subsequence	Medium	LongestIncreasingSubsequence.java	DP Bottom-up
322	Coin Change	Medium	CoinChange.java	DP Bottom-up
474	Ones and Zeroes	Medium	OnesAndZeroes.java	DP
2266	Count Number of Texts	Medium	CountNumberOfTexts.java	DP
2369	Check if There is a Valid Partition For The Array	Medium	CheckIfThereIsAValidPartitionForTheArray.java	DP

Heap (Priority Queue)

No.	Title	Difficulty	Solution	Idea
347	Top K Frequent Elements	Medium	TopKFrequentElements.java	Priority Queue
692	Top K Frequent Words	Medium	TopKFrequentWords.java	Priority Queue
778	Swim in Rising Water	Hard	SwimInRisingWater.java	Priority Queue
1642	Furthest Building You Can Reach	Medium	FurthestBuildingYouCanReach.java	Priority Queue

Stack

No.	Title	Difficulty	Solution	Idea
32	Longest Valid Parentheses	Hard	LongestValidParentheses.java	Stack
316	Remove Duplicate Letters	Medium	RemoveDuplicateLetters.java	Stack
394	Decode String	Medium	DecodeString.java	Two Stack
921	Minimum Add to Make Parentheses Valid	Medium	MinimumAddToMakeParenthesesValid.java	Balance
1047	Remove All Adjacent Duplicates In String	Easy	RemoveAllAdjacentDuplicatesInString.java	Use Stack or StringBuilder.
1249	Minimum Remove to Make Valid Parentheses	Medium	MinimumRemoveToMakeValidParentheses.java	Using Stack to save '('

Math

No.	Title	Difficulty	Solution	Idea
171	Excel Sheet Column Number	Easy	ExcelSheetColumnNumber.java	Iteration
191	Number of 1 Bits	Easy	LongestValidParentheses.java	n & (n - 1)
202	Happy Number	Easy	HappyNumber.java	HashSet | fast-slow pointer

Greedy

No.	Title	Difficulty	Solution	Idea
134	Gas Station	Medium	GasStation.java	Greedy
135	Candy	Hard	Candy.java	From left to right, then form right to left.
179	Largest Number	Medium	LargestNumber.java	Greedy and Sort
376	Wiggle Subsequence	Medium	WiggleSubsequence.java	detect the change.
1647	Minimum Deletions to Make Character Frequencies Unique	Medium	MinimumDeletionsToMakeCharacterFrequenciesUnique.java	Greedy | HashSet | Sort
1710	Maximum Units on a Truck	Easy	MaximumUnitsOnATruck.java	select Greatest first.

Trie

No.	Title	Difficulty	Solution	Idea
1268	Search Suggestions System	Medium	SearchSuggestionsSystem.java	Trie | Binary Search