Introduction

Palindrome Linked List means checking whether linked list reads the same forward and backward.

The task is to:

  • find middle node
  • reverse second half
  • compare both halves

Example:

Input:1 -> 2 -> 2 -> 1

Output:
True

Explanation:

Forward:1 2 2 1

Backward:
1 2 2 1

Both are same. So linked list is palindrome.

This problem is one of the most important applications of:

Fast and Slow Pointer Technique

Constraints

 1 <= Number of Nodes <= 10^5-10^9 <= Node Value <= 10^9

Approach 1 : Brute Force (Using Array)

Explanations:

Explanation:

The idea is:

  • store linked list values in array
  • compare from both ends
  • check palindrome

Steps:

  1. Traverse linked list.
  2. Store values in array.
  3. Compare left and right.
  4. Return result.

This approach works but:

  • uses extra memory

So in-place reversal is preferred.

Dry Run

Input:1 -> 2 -> 2 -> 1

Array:
[1,2,2,1]

Compare:
1 == 1
2 == 2
Palindrome exists

Practice :

Complexity Analysis :

Time Complexity:- O(n)Explanation :
Each node is visited once.

Space Complexity:- O(n)
Explanation :
Extra array is used.

Approach 2 : Optimal Solution(Using Fast & Slow Pointer)

Explanations:

Explanation:

This is the most optimized and interview-preferred solution.

The idea is:

  • find middle node
  • reverse second half
  • compare both halves

This avoids extra memory usage.

Dry Run

Input:1 -> 2 -> 2 -> 1

Middle:
2

Reverse second half:
1 -> 2

Compare:
1 == 1
2 == 2
Palindrome exists

Practice :

Complexity Analysis :

Time Complexity:- O(n)
Explanation :
Each node is visited once.

Space Complexity:- O(1)
Explanation :
No extra space is used.

Why This Problem is Important

This problem builds the foundation for:

  • Fast & Slow Pointer
  • Linked List reversal
  • Pointer manipulation
  • In-place algorithms
  • Efficient comparison logic

Real-World Applications

Palindrome checking concepts are used in:

  • Data validation
  • DNA sequence analysis
  • Pattern matching
  • Text processing systems
  • Symmetry detection systems

Common Beginner Mistakes

  • Incorrect middle node detection
  • Losing second half reference
  • Wrong reversal logic
  • Infinite loops
  • Incorrect comparisons

Interview Tip

Interviewers often expect:

  • Fast & Slow Pointer optimization
  • in-place reversal logic
  • O(1) space solution
  • proper comparison handling

Always explain:

  • why second half is reversed
  • how both halves are compared efficiently

Related Questions

  • Reverse Linked List
  • Reverse Linked List II
  • Reverse Nodes in K Group
  • Linked List Cycle
  • Middle of Linked List

Final Takeaway

The Palindrome Linked List problem is one of the most important linked list interview problems.

It teaches:

  • linked list reversal
  • pointer manipulation
  • efficient comparison
  • in-place algorithms

Understanding this problem builds a strong foundation for:

  • advanced linked list problems
  • pointer-based algorithms
  • interview-level data structure questions.