Pattern this teaches:Fast and Slow Pointers
Palindrome Linked List
An easy Linked List interview guide. Task statement, worked examples, intuition, and step-by-step solutions.
- Topic
- Linked List
- Sheets
- 3
- Core for
- 4 roles
The problem
Given the head of a linked list, determine whether it is a palindrome.
Example 1
- Input
- [1,3,3,1]
- Output
- true
Example 2
- Input
- [2,5,5,3,2]
- Output
- false
Example 3
- Input
- [1]
- Output
- true
Constraints
- 0 <= n <= 10^5
- -10^5 <= Node.val <= 10^5
How to think about it
Updated 2026-09-29Symmetry demands comparing outermost pairs toward the center, but singly linked lists cannot be navigated in reverse. Halving the list and reversing only the rear half creates two independent chains that can be traversed forward side-by-side to compare values.
Approaches, worst first
Copy to array buffer
time O(n) · space O(n)
Extract all values into an array list, then check palindrome properties using standard two pointers converging from both boundaries. Trivial to write, but allocates heap memory proportional to the list size.
Split, reverse second half, and compareWrite this one
time O(n) · space O(1)
Find the midpoint using fast and slow pointers, reverse the list starting after the midpoint, and compare node values sequentially against the front half. Optionally restore the second half before returning to avoid mutating caller data.
Where people lose marks · 3
- Forgetting to terminate the front half before the reversed second half causes comparison loops to overshoot on odd-length lists.
- Comparing node references instead of node values tests whether instances are identical rather than checking if values mirror symmetrically.
- An empty list or single-node list must return true without triggering null dereferences during midpoint splitting.
The theory behind it
Linked List — the ground this problem stands on. All Linked List problems
What Linked List is
A linked list is a chain of separate cargo cars connected by coupling hooks, scattered anywhere across memory rather than sitting in a tidy contiguous row. Each car, called a node, holds a single piece of data and a pointer directing traffic to the address of the next car in line. Because nodes connect only by directional links, jumping straight to the tenth car is impossible without walking past the first nine.
When to reach for it
Choose a linked list when a problem requires frequent insertions and deletions at known positions without shifting whole blocks of surrounding memory. Problems mentioning pointer splicing, reversing subsequences in place, merging sorted streams, or detecting cycles in linear chains strongly point here. It is ideal when total capacity is unpredictable and memory allocation must happen one individual node at a time.
How the pattern works
Think in terms of pointer rewiring before dereferencing. Keep a dummy head node pointing to the start of the list so modifications to the initial item do not require separate edge logic. Always save references to neighboring nodes into temporary variables before cutting or redirecting forward links. When diagnosing loops or locating middle nodes, advance two references simultaneously at differing velocities so traversal completes without supplementary storage.
More Linked List problems
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