Stop 21 · Data Structures & AlgorithmsTopics · 3/17

Linked List

In short

You'll rarely use a linked list in a React app, but interviewers love them because they test whether you can move pointers around without losing anything.

A linear data structure of nodes linked via pointers rather than contiguous memory — a classic interview topic for pointer manipulation practice.

The jaadu reads a treasure hunt clue next to a dotted path leading to a flower pot.

Think of it like this

A college fest treasure hunt

Each clue tells you only where the next clue is hidden. To reach clue 7, you must follow clues 1 to 6. Lose one clue and the rest are gone. That's a linked list: each node knows only its next node.

Watch it happen

Reversing 1 → 2 → 3

1 → 2

curr

2 → 3

3 → null

1/4prev is null and curr is node 1. Save next (node 2) before touching anything.

The big ideas

What you need to know

Nodes point to the next

Each node stores a value and a next pointer. The last node points to null.

No jumping to index k

Reaching the k-th node means walking from the head, so access is O(n).

Cheap insert and delete

If you already hold the node before the spot, inserting or removing is O(1). Just rewire pointers.

Slow and fast pointers

One moves 1 step, the other 2. Use it to find the middle or detect a cycle.

See it in code

Walk through the code

1// node: { val, next }2function reverseList(head) {3  let prev = null;4  let curr = head;5 6  while (curr) {7    const next = curr.next; // save before breaking the link8    curr.next = prev;       // flip the arrow9    prev = curr;10    curr = next;11  }12  return prev; // the new head13}

1/4prev starts as null because the old head will become the tail.

Each node is visited once, so time is O(n). Only three pointers are used, so extra space is O(1).

Save yourself the pain

Common mistakes

Oops

Changing curr.next before saving it, and losing the rest of the list.

Do this instead

Always store next first. Draw the boxes and arrows on paper before you code.

Oops

Writing lots of special cases for deleting the head node.

Do this instead

Add a dummy node before the head. Then the head is just a normal node, and you return dummy.next.

Your mission

Pointer practice

~35 min

Quick check

How long does it take to read the 500th node of a singly linked list?

Go deeper

1Implement a singly linked list from scratch (insert, delete, traverse).
2Solve classic problems: reverse a linked list, detect a cycle, find the middle node.
3Learn the 'fast and slow pointer' (tortoise and hare) technique.