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DSA Foundations

Build your foundations: arrays, hashing, linked lists, stacks & queues, recursion, and trees — the data structures every interview builds on.

Islands
5
Lessons
44
Exercises
275
Complete codeDebugCode orderFill blankMultiple choiceTapTrace

Start the first lesson

  1. Island 1

    Arrays & Hashing

    1. Array Traversal & Indexing6 exercises, 6 min
    2. In-Place Modification6 exercises, 7 min
    3. Array Basics CheckpointCheckpoint7 exercises, 8 min
    4. Hash Sets6 exercises, 7 min
    5. Frequency Counting6 exercises, 7 min
    6. Anagram Check6 exercises, 7 min
    7. Two Sum with a Hash Map6 exercises, 7 min
    8. Arrays & Hashing ReviewReview6 exercises, 6 min
    9. Boss: Two SumBoss8 exercises, 8 min
  2. Island 2

    Linked Lists

    1. Nodes & Traversal6 exercises, 6 min
    2. Insert & Delete6 exercises, 7 min
    3. Linked List Basics CheckpointCheckpoint7 exercises, 8 min
    4. Reverse a Linked List6 exercises, 7 min
    5. Find the Middle6 exercises, 7 min
    6. Cycle Detection6 exercises, 7 min
    7. Merge Two Sorted Lists6 exercises, 7 min
    8. Linked Lists ReviewReview6 exercises, 8 min
    9. Boss: Reverse a Linked ListBoss8 exercises, 10 min
  3. Island 3

    Stack & Queue Basics

    1. Stacks6 exercises, 6 min
    2. Queues6 exercises, 6 min
    3. Stacks & Queues CheckpointCheckpoint7 exercises, 8 min
    4. Valid Parentheses5 exercises, 5 min
    5. Min Stack5 exercises, 5 min
    6. Queue from Two Stacks6 exercises, 5 min
    7. Stack & Queue ReviewReview6 exercises, 6 min
    8. Boss: Valid ParenthesesBoss8 exercises, 8 min
  4. Island 4

    Recursion

    1. What Is Recursion?6 exercises, 6 min
    2. Base Case6 exercises, 6 min
    3. Recursive Call6 exercises, 7 min
    4. Return Value6 exercises, 7 min
    5. Call Stack6 exercises, 7 min
    6. Recursion Basics CheckpointCheckpoint6 exercises, 8 min
    7. Fibonacci Number5 exercises, 5 min
    8. Recursion ReviewReview6 exercises, 6 min
    9. Boss: Pow(x, n)Boss8 exercises, 8 min
  5. Island 5

    Tree Basics

    1. Binary Tree Structure6 exercises, 6 min
    2. Empty Tree Base Case6 exercises, 7 min
    3. Depth-First Traversal6 exercises, 7 min
    4. Breadth-First Traversal6 exercises, 7 min
    5. Binary Search Tree Search6 exercises, 7 min
    6. Maximum Depth7 exercises, 8 min
    7. Tree Patterns CheckpointCheckpoint6 exercises, 6 min
    8. Tree Basics ReviewReview6 exercises, 6 min
    9. Boss Battle: Maximum DepthBoss8 exercises, 10 min