Java: From Beginning to Medium Level

My Java practice repository. It is a running log of what I have worked through while learning the language: small programs covering the basics, notes on the collections framework, and a growing pile of LeetCode solutions.

Nothing here is a library or a framework. Each file is self contained, has a main with the example cases from the problem, and can be run on its own.

Layout

Java_DailyLearningPractice/   day to day language practice
LeetcodePractice/             one file per LeetCode problem
tools/build_index.py          regenerates the index below

Java_DailyLearningPractice/ is the basics: syntax, loops, a calculator, and a short tour of ArrayList, HashSet, HashMap, ArrayDeque, LinkedList and PriorityQueue. A few classic problems (longest common prefix, longest palindromic substring, Pascal's triangle) ended up here before I started the LeetCode folder.

LeetcodePractice/ is everything else, indexed by topic below.

Running a solution

Both folders are packages, so compile from the repository root:

javac -d out LeetcodePractice/TwoSum.java
java -cp out LeetcodePractice.TwoSum

Or build everything at once:

javac -d out LeetcodePractice/*.java Java_DailyLearningPractice/*.java

Solutions by topic

240 solutions, grouped by the main technique each one uses.

Arrays | Strings | Hashing | Two pointers | Sliding window | Prefix sum | Stack and queue | Binary search | Sorting and greedy | Heap | Linked lists | Trees | Graphs, BFS and DFS | Backtracking | Dynamic programming | Math and bit manipulation | Design

Arrays

Problem	Solution
Array Partition	ArrayPartition.java
Bus Shortest Distance	BusShortestDistance.java
Contains Duplicate	ContainsDuplicate.java
Distance Between Arrays	DistanceBetweenArrays.java
Duplicate Zeros	DuplicateZeros.java
Fair Candy Swap	FairCandySwap.java
Height Checker	HeightChecker.java
Kids with Candy	KidsWithCandy.java
Kth Missing Positive	KthMissingPositive.java
Large Group Positions	LargeGroupPositions.java
Largest Odd Number	LargestOddNumber.java
Majority Element	MajorityElement.java
Max Consecutive Ones	MaxConsecutiveOnes.java
Minimum Start Value	MinimumStartValue.java
Monotonic Array	MonotonicArray.java
Move Zeroes	MoveZeroes.java
Remove Element	RemoveElement.java
Replace Elements with Greatest on Right	ReplaceElementsWithGreatestOnRight.java
Slowest Key Press	SlowestKeyPress.java
Smallest Number	SmallestNumber.java
Sorted Squares	SortedSquares.java
Special Array	SpecialArray.java
Third Maximum Number	ThirdMaximumNumber.java
Three Consecutive Odds	ConsecutieOdds.java
Triplet Sequence	TripletSequence.java
Valid Mountain	ValidMountain.java

Strings

Problem	Solution
Anagram Checker	AnagramChecker.java
Are Almost Equal	AreAlmostEqual.java
Attendance Award	AttendanceAward.java
Balanced String	BalancedString.java
Balanced String Placed	BalancedStringPlaced.java
Binary String With at Most One Segment of Ones	CheckOnesSegment.java
Buddy Strings	BuddyStrings.java
Chessboard	Chessboard.java
Consecutive Char	ConsecutiveChar.java
Count Asterisks	CountAsterisks.java
Count Binary Substrings	CountBinarySubstrings.java
Coupon Validator	CouponValidator.java
Detect Capital	DetectCapital.java
Find the Difference	FindTheDifference.java
Find the Index of the First Occurrence in a String	FirstOccurence.java
Generate the String	GenerateTheString.java
Goat Latin	GoatLatin.java
Greatest Common Divisor of Strings	StringDivisor.java
Has Valid Substring	HasValidSubstring.java
Is Long Pressed Name	IsLongPressedName.java
Is Prefix String	IsPrefixString.java
Length of Last Word	LengthOfLastWord.java
License Key Formatting	LicenseKeyFormatting.java
Max Score Split	MaxScoreSplit.java
Max Vowel Consonant Frequency	MaxVowelConsonantFrequency.java
Minimum Moves to Convert String	MinimumMovesToConvertString.java
Number of Different Integers in a String	NumDifferenceInt.java
Number of Segments	NumberOfSegments.java
Occurrences After Bigram	OccurrencesAfterBigram.java
Rearrange Spaces	RearrangeSpaces.java
Reformat Phone Number	ReformatPhoneNumber.java
Reformat String	ReformatString.java
Remove Palindrome	RemovePalindrome.java
Reorder String	ReorderString.java
Repeated Substring Pattern	RepeatedSubstringPattern.java
Reverse Only Letters	ReverseOnlyLetters.java
Reverse Prefix	ReversePrefix.java
Reverse String II	ReverseStringII.java
Reverse Words	ReverseWords.java
Reverse Words in String	ReverseWordsInString.java
Second Highest	SecondHighest.java
Shortest Distance to Char	ShortestDistanceToChar.java
Shuffle String	ShuffleString.java
Sorting the Sentence	SortingTheSentence.java
String Compressor	StringCompressor.java
String Matching in Array	StringMatchingInArray.java
String Reverse	StringReverse.java
String Rotation Check	StringRotationCheck.java
Strong Password Checker	StrongPasswordChecker.java
Sum of Digits of String After Convert	GetLuck.java

Hashing

Problem	Solution
Array Difference	ArrayDifference.java
Array Intersection	ArrayIntersection.java
Check If Pangram	CheckIfPangram.java
Close Strings Checker	CloseStringsChecker.java
Common Characters	CommonCharacters.java
Count Characters	CountCharacters.java
Destination City	DestinationCity.java
Distribute Candies	DistributeCandies.java
Equal Row Column Pairs	EqualRowColumnPairs.java
First Unique Char	FirstUniqueChar.java
Good Pairs	GoodPairs.java
Good String Check	GoodStringCheck.java
Happy Number	HappyNumber.java
Intersection of Two Arrays II	IntersectionOfTwoArraysII.java
Is Path Crossing	IsPathCrossing.java
Isomorphic Strings	IsomorphicStrings.java
Jewels and Stones	JewelsAndStones.java
Longest Palindrome	LongestPalindrome.java
Longest Substring Between Equal Chars	LongestSubstringBetweenEqualChars.java
Lucky Integer	LuckyInteger.java
Max Operations on Array	MaxOperationsOnArray.java
Minimum Index Sum of Two Lists	MinimumIndexSumOfTwoLists.java
Most Common Word	MostCommonWord.java
Odd String Difference	OddStringDifference.java
Rank Transform	RankTransform.java
Ransom Note Checker	RansomNoteChecker.java
Reformat Date	ReformatDate.java
Roman to Integer	RomanToInteger.java
Set Mismatch	SetMismatch.java
Shortest Completing Word	ShortestCompletingWord.java
Shortest Sub Array Degree	ShortestSubArrayDegree.java
Sort Frequency	SortFrequency.java
Sum of Unique Elements	SumOfUniqueElements.java
Two Sum	TwoSum.java
Unique Emails	UniqueEmails.java
Unique Occurrences	UniqueOccurrences.java
Word Pattern Match	WordPatternMatch.java

Two pointers

Problem	Solution
3Sum	ThreeSum.java
3Sum Closest	ThreeSumCloset.java
Container with Most Water	ContainerWithMostWater.java
Merge Alternately	MergeAlternately.java
Merge Sorted Arrays	MergeSortedArrays.java
Min Deletion Size	MinDeletionSize.java
Remove Duplicates	RemoveDuplicates.java
Reverse Vowels	ReverseVowels.java
Subsequence Checker	SubsequenceChecker.java
Valid Palindrome	ValidPalindrome.java
Valid Palindrome II	ValidPalindromeII.java

Sliding window

Problem	Solution
K Beauty of Number	KBeautyOfNumber.java
Longest Subarray After Deleting One	LongestSubarrayAfterDeletingOne.java
Longest Substring Without Repeating Characters	LongestSubstringwithoutRepeating.java
Max Avg Sub Array	MaxAvgSubArray.java
Max Consecutive Ones III	MaxConsecutiveOnesIII.java
Max Vowels	MaxVowels.java
Permutation from String	PermutationFromString.java

Prefix sum

Problem	Solution
Find Pivot Index	PivotIndexFinder.java
Highest Altitude	HighestAltitude.java
Product of Array Except Self	ProductOfArrayExceptSelf.java
Running Sum	RunningSum.java

Stack and queue

Problem	Solution
Asteroid Collision	AsteroidCollision.java
Backspace	Backspace.java
Baseball Game	BaseballGame.java
Daily Temperatures	DailyTemperatures.java
Decode String	DecodeString.java
Dota2 Senate	DotaSenate.java
Make String Great	MakeStringGreat.java
Maximum Nesting Depth of the Parentheses	MaxDepth.java
Next Greater Element	NextGreaterElement.java
Remove All Adjacent Duplicates in String	RemoveDuplicatess.java
Remove Outer Parentheses	RemoveOuterParentheses.java
Remove Stars from String	RemoveStarsFromString.java
Stack (practice with the collection)	StackCollection.java
Valid Parentheses	ValidParanthesis.java

Binary search

Problem	Solution
Binary Search	BinarySearch.java
Guess Game	GuessGame.java
Koko Eating Bananas	KokoEatingBananas.java
Median of Two Sorted Arrays	MedianOfTwoSortedArrays.java
Peak Element	PeakElement.java
Search Insert	SearchInsert.java
Search Range	SearchRange.java
Search Suggestion System	SearchSuggestionSystem.java
Square Root	SquareRoot.java
Successful Pairs	SuccessfulPairs.java

Sorting and greedy

Problem	Solution
Assign Cookies	AssignCookies.java
Jump Game	JumpGame.java
Largest Num	LargestNum.java
Maximum Units on Truck	MaximumUnitsOnTruck.java
Minimum Arrows to Burst Balloons	MinimumArrowsToBurstBalloons.java
Non Overlapping Intervals	NonOverlappingIntervals.java
Relative Ranks	RelativeRanks.java
Triangle Perimeter	TrianglePerimeter.java

Heap

Problem	Solution
Hiring Workers	HiringWorkers.java
Kth Largest Element	KthLargestElement.java
Max Score Subsequence	MaxScoreSubsequence.java

Linked lists

Problem	Solution
Delete Middle Node	DeleteMiddleNode.java
Maximum Twin Sum of a Linked List	PairSum.java
Merge Two Lists	MergeTwoLists.java
Odd Even Linked List	OddEvenLinkedList.java
Reverse Linked	ReverseLinked.java

Trees

Problem	Solution
Delete Node BST	DeleteNodeBST.java
Good Nodes in Binary Tree	GoodNodesInBinaryTree.java
Leaf Similar Trees	LeafSimilarTrees.java
Longest Zig Zag Path	LongestZigZagPath.java
Lowest Common Ancestor	LowestCommonAncestor.java
Max Depth Binary Tree	MaxDepthBinaryTree.java
Max Level Sum Binary Tree	MaxLevelSumBinaryTree.java
Path Sum III	PathSumIII.java
Right Side View	RightSideView.java
Root to Leaf Paths	RootToLeafPaths.java
Search BST	SearchBST.java

Graphs, BFS and DFS

Problem	Solution
Evaluate Division	EvaluateDivision.java
Flower Planting	FlowerPlanting.java
Island Perimeter	IslandPerimeter.java
Nearest Exit in Maze	NearestExitInMaze.java
Number of Provinces	NumberOfProvinces.java
Reorder Routes	ReorderRoutes.java
Rotting Oranges	RottingOranges.java
Town Judge	TownJudge.java
Visit All Rooms	VisitAllRooms.java

Backtracking

Problem	Solution
Combination Sum	CombinationSum.java
Combination Sum3	CombinationSum3.java
Count Good Triplets	CountGoodTriplets.java
Letter Combinations	LetterCombinations.java
Word Search	WordSearch.java

Dynamic programming

Problem	Solution
Best Time to Buy and Sell Stock	BestTimeToBuyAndSellStock.java
Edit Distance	EditDistance.java
House Robber	HouseRobber.java
Longest Common Subsequence	LongestCommonSubsequence.java
Min Cost Climbing Stairs	MinCostClimbingStairs.java
Stock with Transaction Fee	StockWithTransactionFee.java
Tiling Domino Tromino	TilingDominoTromino.java
Tribonacci	Tribonacci.java
Unique Paths	UniquePaths.java

Math and bit manipulation

Problem	Solution
Add Binary	AddBinary.java
Add Strings	AddStrings.java
Add to Array Form	AddToArrayForm.java
Binary Date	BinaryDate.java
Bitwise Flip Counter	BitwiseFlipCounter.java
Convert to Base Seven	ConvertToBaseSeven.java
Count Bits	CountBits.java
Day of Year	DayOfYear.java
Days Between Dates	DaysBetweenDates.java
Divide Two Integers	DivideTwoIntegers.java
Excel Column Number	ExcelColumnNumber.java
Excel Column Title	ExcelColumnTitle.java
Fizz Buzz	FizzBuzz.java
Integer to Roman	IntegerToRoman.java
Missing Number	MissingNumber.java
Multiply Strings	MultiplyStrings.java
Palindrome Number	PalindromeNumber.java
Plus One	PlusOne.java
Power	Power.java
Reverse Integer	ReverseInteger.java
Rotate Matrix	RotateMatrix.java
Single Number	SingleNumber.java
Smallest Range I	SmallestRangeI.java
String to Integer (atoi)	MyAtoi.java
Thousand Separator	ThousandSeperator.java

Design

Problem	Solution
Design HashMap	MyHashMap.java
PriorityQueue (practice with the collection)	PriorityQueueCollection.java
Recent Counter	RecentCounter.java
Smallest Infinite Set	SmallestInfiniteSet.java
Stock Spanner	StockSpanner.java
Trie	Trie.java

Regenerating the index

The table above is generated. After adding a solution, add its class name to the topic map in tools/build_index.py and run:

python tools/build_index.py

It warns about any file that is not in the map, so nothing goes missing.