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Scala

686 problems50 sections📅 2026 curriculum

Learn Scala by typing expressive, functional code.

Practice Scala's blend of OO and functional by typing it: pattern matching, case classes, immutable collections, for-comprehensions, and higher-order functions. Each snippet is idiomatic Scala, and an AI tutor explains every line — how a match deconstructs a case class, why immutability simplifies reasoning — so the paradigm and the keystrokes land together.

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Run Scala on your machine

1. Install the toolchain

brew install scala # (needs a JDK)

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

scala hello.scala

Official install docs ↗

progress by section

1Fundamentals0/9start →

val and var Hello, Scala Methods for over a range if as an expression Numeric and boolean types while loop match expression Ranges

2Strings In Depth0/19start →

String basics Change case Reverse capitalize and toList Iterate characters Predicates on text indexOf Interpolators Repeat a string replace Trim whitespace substring and slice StringBuilder Filter characters Multi-line strings padTo split and mkString Count words Extension methods

3Values & References0/16start →

Structural equality val vs var Case class equality Case class copy Immutable collections Option over null Parameters are local Immutable arguments are safe Aliasing Defensive copy eq vs ==lazy val Pass a mutable reference Value vs reference types Identity hash Shared nested object

4Slices & Arrays0/18Unlock →

reverse Array basics contains take and drop distinct Safe access slice a range sorted and sortBy indexOf and indexWhere sortWith splitAt takeWhile and dropWhile updated zipWithIndex sorted with a given Ordering grouped patch sliding

5Data structures0/10Unlock →

List basics Tuples Array Map Set Vector ArrayBuffer Case classes Immutable vs mutable Mutable Map word count

6Iterators0/20Unlock →

7Traits0/20Unlock →

8Generics0/20Unlock →

9Lifetimes0/20Unlock →

10Closures0/20Unlock →

11Pattern Matching0/20Unlock →

12Error Handling0/20Unlock →

13Option & Result0/20Unlock →

14Conversions0/15Unlock →

Value to String Boolean to number Char and code point Chars to String Numeric conversion Parse a String String to List of chars List to Array Int to another base asInstanceOf Float to int truncates Parse with a radix Convert with .to toIntOption given Conversion

15Collection Conversions0/19Unlock →

Flatten nested Lists List to Set List and Vector Map keys to List Map values to List mkString Pairs to a Map Set to sorted List List to SortedSet Unique, keeping order flatMap Frequency map Map keys to a Set Map to List of pairs Sort by a key Map to SortedMap Two Lists to a Map groupBy Invert a Map

16Math0/14Unlock →

Math constants Float functions Integer arithmetic min, max, clamp Rounding Logarithms & exp Min/max of a collection Trigonometry Factorial with BigInt Greatest common divisor hypot & atan2 Primality test Euclidean div & mod Mean, median, stdev

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 Parse key=value config Round-trip write then read Stream lines from a file Drop unparseable rows

20Linear Algebra0/21Unlock →

21Text Processing0/16Unlock →

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

22Date & Time0/12Unlock →

Day of the week Leap year check LocalDate fields Days between dates Duration arithmetic Format a datetime Seconds to HH:MM:SS Instant and epoch Parse a date string Date arithmetic Period between dates Zoned datetime

23Memory0/12Unlock →

Immutable update Swap two values Aliasing a reference Array vs List Immutable copy Reference vs value equality Mutable buffer Reference inequality Structural equality Identity hash Lazy memoization Case class as map key

24Modules0/20Unlock →

25Macros0/20Unlock →

26Testing0/20Unlock →

27Algorithms0/8Unlock →

Greatest common divisor Binary search Linear congruential generator Palindrome check Fibonacci with memoization Fisher-Yates shuffle Merge two sorted arrays Two sum

28Sorting Algorithms0/12Unlock →

Standard library sorts Bubble sort Sort by key and descending Insertion sort Selection sort Counting sort Custom Ordering 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 interpolation Case classes and copy Collection pipelines Extension methods Fold for-comprehensions Match with guards Option over null given / using

31OOP & Traits0/8Unlock →

Case class Class with fields Encapsulation Companion object Composition Abstract class & override Enums Sealed hierarchy

32Trait Objects0/14Unlock →

List of a trait type Subtype polymorphism Default method dispatch List of functions Handler registry Return a trait type Sealed trait dispatch Sum over a trait type Enum with methods Bounded generic vs trait param Supertrait Upcast and match Given instances registry Visitor over a hierarchy

33Design Patterns0/14Unlock →

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

34Async0/3Unlock →

A Future block Future for-comprehension Many futures

35File I/O0/6Unlock →

Read a file Write a file Append with a PrintWriter Count lines Read lines with Source Buffered writing

36Filesystem0/18Unlock →

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

37Networking0/16Unlock →

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

38Concurrency0/7Unlock →

A Future Spawn a thread An ExecutorService Future.sequence Atomic counter Parallel stream synchronized

39Unsafe & FFI0/14Unlock →

A raw Array asInstanceOf classOf isInstanceOf Handling null System.arraycopy ByteBuffer Low-level memory Byte order Bytes as a float Reinterpret bits Reflect a field Invoke a method Unsigned bytes

40Commands0/14Unlock →

Command-line arguments First arg and the rest Env var with a default Read an environment variable Current directory Iterate all env vars Exit with a code System properties Pass env to a child Run a command Spawn and wait Command exit status Read a line of stdin Find a program 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