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Kotlin

706 problems50 sections📅 2026 curriculum

Learn Kotlin by typing concise, idiomatic code.

Practice Kotlin's concise style by typing it: data classes, null safety with ?. and ?:, extension functions, sealed classes, and coroutines for async work. Each snippet is idiomatic Kotlin, and an AI tutor explains every line — how the compiler enforces null safety, when a scope function fits — so you write less boilerplate and type it fluently.

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Run Kotlin on your machine

1. Install the toolchain

brew install kotlin

2. Save your code as hello.kt, then compile & run

kotlinc hello.kt -include-runtime -d hello.jar && java -jar hello.jar

Official install docs ↗

progress by section

1Fundamentals0/15start →

Hello, Kotlin Numeric types String templates val and var for over a range Functions if is an expression Lists and arrays when on a value while loop Expression functions downTo and step when without a subject Loop with an index Nullable types

2Strings In Depth0/17start →

Templates Case and trimming Strings are immutable Repeat Iterate characters Classify characters joinToString Iterate lines padStart and padEnd Replace text Search a string slice with a range split substring substringBefore StringBuilder Encode to bytes

3Values & References0/15start →

val vs var Compile-time constants Parameters are local Nullable and elvis Structural equality val fixes the binding Aliasing Data class copy Drop the nulls An independent copy let on a nullable Mutating a passed collection Referential equality Smart cast on null check Read-only view

4Slices & Arrays0/17Unlock →

Arrays Membership reversed distinct first, last, getOrNull slice with ranges Sort in place sorted returns new subList take and drop filterIndexed flatten indexOf sortedBy a key binarySearch chunked windowed

5Data structures0/14Unlock →

List basics List operations Maps Pair and Triple Sets associateWith Data classes Word count Set algebra ArrayDeque groupingBy and groupBy Sorted maps A generic stack Sealed classes

6Iterators0/20Unlock →

7Traits0/20Unlock →

8Generics0/20Unlock →

9Lifetimes0/20Unlock →

10Closures0/20Unlock →

11Pattern Matching0/20Unlock →

12Error Handling0/20Unlock →

13Option & Result0/20Unlock →

14Conversions0/15Unlock →

String to Double String to Int Value to String Char to code point CharArray to String Code point to Char Safe parse Range to List List to Set List to Array Bit operations Numeric conversions Parse with a radix Int to a radix string String and ByteArray

15Collection Conversions0/19Unlock →

Flatten nested lists Map keys to List Set to sorted List sorted and reversed Pairs to a Map To a mutable List List to Set Map values to List associate to a Map associateBy associateWith Distinct, keeping order Frequency map flatMap Set operations zip and unzip distinctBy Group into a Map Invert a Map

16Math0/14Unlock →

Math constants Float functions Integer arithmetic min, max, clamp Rounding Average of a list Logarithms & exp abs diff & sign Trigonometry Bit math Comparing floats Floored div & mod hypot & atan2 Min/max of a list

17Encoding0/22Unlock →

18Random0/20Unlock →

19Data I/O0/18Unlock →

Parse CSV into rows Tab-separated values Build a CSV string Select one column CSV into a number matrix Split off the header Whitespace-separated matrix Read a data file Serialize records to CSV Skip blanks and comments Print an aligned table Write CSV to a file Write a matrix as text Append a row Stream lines lazily Parse key=value config Round-trip write then read Drop unparseable rows

20Linear Algebra0/21Unlock →

21Text Processing0/16Unlock →

Word count Capitalize the first letter Count occurrences Strip all whitespace Replace text Reverse word order Count vowels Zero-pad a number Pad and align Character frequency Index each char Longest word Number the lines Title case Truncate with an ellipsis Levenshtein distance

22Date & Time0/13Unlock →

Leap year check Local date fields Local date and time Duration arithmetic UNIX timestamp now Format a date-time Seconds to HH:MM:SS Date arithmetic Days between dates Epoch seconds to a date Parse a date string Period between dates Zoned date-time

23Memory0/12Unlock →

Copying an array === vs ==List structural equality String literals pool Aliased mutation Arrays vs lists copy() is shallow A manual deep copy identityHashCode String interning A WeakReference Integer caching

24Modules0/20Unlock →

25Macros0/20Unlock →

26Testing0/20Unlock →

27Algorithms0/8Unlock →

Greatest common divisor Binary search Palindrome check Fibonacci with memoization Merge two sorted arrays Partition around a pivot Shuffle with an LCG Two sum

28Sorting Algorithms0/12Unlock →

sorted & sortedBy Bubble sort sortedDescending Insertion sort Selection sort sortWith comparator Counting sort Merge sort Shell sort Heapsort Quicksort Radix sort

29Expression Parsing0/6Unlock →

Evaluate postfix (RPN)Tokenize an expression Evaluate infix Recursive-descent calculator Right-associative power Shunting-yard

30Idioms0/9Unlock →

String templates Data class destructuring takeIf with Elvis joinToString Collection pipelines groupingBy counts Scope functions when expression Lazy sequences

31OOP & Traits0/8Unlock →

Class with init Encapsulation & visibility Abstract class Composition & polymorphism Data class Inheritance Companion & object Sealed class

32Trait Objects0/14Unlock →

Take an interface Default over polymorphism Safe cast with as?Enum with abstract method Factory returns an interface A fun interface (SAM)Generic with an interface bound is checks & smart cast List of an interface A list of functions A handler registry Sealed + when dispatch Sum over an interface list when as an expression

33Design Patterns0/14Unlock →

Adapter Lazy Initialization Prototype (clone)Singleton Builder (apply DSL)Dependency Injection Factory Method Observer (pub/sub)Strategy Command Decorator Object Pool State Actor (message passing)

34Async0/8Unlock →

A custom executor supplyAsync thenApply allOf thenCombine exceptionally orTimeout thenCompose

35File I/O0/6Unlock →

Read a file Read all lines Write a file Append to a file kotlin.io.path API Buffered read and write

36Filesystem0/18Unlock →

Does a path exist?Copy a file Files vs directories List a directory Create nested directories Rename or move Build a path Change the extension Delete a whole tree File size Path components Sorted listing Create a temp directory Filter by extension Last modified time Walk with java.nio Total size of a tree Walk the tree

37Networking0/16Unlock →

Classify an IP address Parse the request line Route by path A socket address Build an HTTP response Bind an ephemeral port Read a line from a socket Resolve a hostname Half-close a connection TCP client Read timeout UDP receive UDP send A thread per connection TCP echo server + client Host a web page

38Concurrency0/15Unlock →

Spawn a thread A Thread from a Runnable Virtual thread per task Start a virtual thread AtomicInteger Callable and Future A thread pool @Synchronized methods Producer/consumer ConcurrentHashMap CountDownLatch ReentrantLock Scoped values Semaphore synchronized block

39Unsafe & FFI0/14Unlock →

Raw byte array Raw char array System.arraycopy ByteBuffer Identity hash Reverse bytes Byte order Direct ByteBuffer Reinterpret float bits Reflect a private field Reflect a private method WeakReference Cleaner A MethodHandle

40Commands0/14Unlock →

Command-line arguments Number each argument Env var with a default Read an environment variable JVM system property Current directory Iterate all env vars Exit with a code Read a line of input Pass env to a child Run a command Command exit code Read all of stdin which: find on PATH

41Array0/10Unlock →

Contains Duplicate Two Sum Best Time to Buy and Sell Stock Maximum Subarray Container With Most Water Find Minimum in Rotated Sorted Array Product of Array Except Self Maximum Product Subarray Search in Rotated Sorted Array 3Sum

42Binary0/5Unlock →

Missing Number Number of 1 Bits Counting Bits Reverse Bits Sum of Two Integers

43String0/10Unlock →

Valid Anagram Valid Palindrome Valid Parentheses Encode and Decode Strings Group Anagrams Longest Substring Without Repeating Characters Longest Repeating Character Replacement Longest Palindromic Substring Palindromic Substrings Minimum Window Substring

44Matrix0/4Unlock →

Rotate Image Set Matrix Zeroes Spiral Matrix Word Search

45Interval0/5Unlock →

Meeting Rooms Merge Intervals Insert Interval Non-overlapping Intervals Meeting Rooms II

46Linked List0/6Unlock →

Reverse Linked List Linked List Cycle Merge Two Sorted Lists Remove Nth Node From End Merge K Sorted Lists Reorder List

47Tree0/14Unlock →

Maximum Depth of Binary Tree Invert Binary Tree Same Tree Lowest Common Ancestor of a BST Binary Tree Level Order Traversal Subtree of Another Tree Implement Trie (Prefix Tree)Kth Smallest Element in a BST Validate Binary Search Tree Add and Search Word Construct Binary Tree from Preorder and Inorder Binary Tree Maximum Path Sum Serialize and Deserialize Binary Tree Word Search II

48Graph0/8Unlock →

Number of Connected Components Number of Islands Clone Graph Course Schedule Longest Consecutive Sequence Graph Valid Tree Alien Dictionary Pacific Atlantic Water Flow

49Heap0/3Unlock →

Kth Largest Element in an Array Top K Frequent Elements Find Median from Data Stream

50Dynamic Programming0/11Unlock →

Climbing Stairs House Robber Unique Paths Coin Change House Robber II Jump Game Combination Sum IV Decode Ways Longest Common Subsequence Longest Increasing Subsequence Word Break