Chapter 02 - Topic 2.4

Linked List Reversal Techniques

Reversing a linked list is a pointer problem, not a swapping problem. The main idea is to turn every next pointer backward without losing access to the remaining nodes.

reverse linked listlinked list reversal techniquesiterative reverse linked listrecursive reverse linked listpointer flip linked list

Definition

Linked-list reversal means changing pointer directions so the list runs in the exact opposite order from its original arrangement.

Core Idea

Reversal means changing the direction of every next pointer so the list runs backward.

The iterative pattern usually uses prev, current, and nextTemp.

The same pointer-flip idea extends to recursive reversal, k-group reversal, and reversing between positions m and n.

Detailed Explanation

Linked-list reversal is one of the best exercises for learning pointer discipline. Every line matters because one careless overwrite can disconnect the rest of the structure permanently.

The iterative solution is the foundation. Once you understand how prev, curr, and next move together, the other variants become easier to reason about because they still depend on the same save-flip-advance pattern.

Advanced reversal questions usually differ in scope, not in core mechanism. Full reversal affects the whole list, reverse-between affects a zone, and k-group reversal applies the same idea repeatedly to fixed-size chunks.

Iterative three-pointer reversal

JavaScript

function reverse(head) {
  let prev = null;
  let current = head;

  while (current !== null) {
    const nextTemp = current.next;
    current.next = prev;
    prev = current;
    current = nextTemp;
  }

  return prev;
}

Explanation

nextTemp stores the original next node before the link is changed. current.next is then flipped to point backward, and the pointers move one step forward. At the end, prev becomes the new head.

Output

For 1 -> 2 -> 3 -> 4 -> 5 -> null, the result becomes 5 -> 4 -> 3 -> 2 -> 1 -> null

Real-life Example

If a one-way route map needs to be traversed in the opposite direction, every arrow must be flipped carefully without breaking the chain.

When To Use It

Use full iterative reversal for production-grade complete list reversal.
Use recursive reversal when you want an elegant interview explanation and the list is not extremely large.
Use k-group and reverse-between when the problem asks for partial or patterned reversal.

Common Mistakes

Not storing the next node before flipping current.next.
Returning the old head instead of the new head after reversal finishes.
Breaking reconnection logic in k-group or reverse-between problems.

Key Takeaways

Always save curr.next before overwriting it.
The prev-current-next pattern is the core reversal template.
Iterative, recursive, k-group, and reverse-between are all variations of the same pointer-flip idea.

Practice Ideas

Reverse a full singly linked list iteratively.
Reverse a list recursively after mastering the iterative version.
Reverse nodes in groups of k and then reverse only between positions m and n.

Frequently Asked Questions

Why do we need a temporary next pointer?

Because once current.next is changed, the original forward link is lost unless it was saved first.

Is linked-list reversal in-place?

Yes, the standard iterative solution reverses the list by only changing pointers and uses O(1) extra space.

Which reversal technique is best for production code?

The iterative three-pointer technique is usually best for production because it is O(n), O(1), and avoids recursion depth issues.

Linked List Reversal Techniques

Definition

Core Idea

Detailed Explanation

Iterative three-pointer reversal

When To Use It

Common Mistakes

Key Takeaways

Practice Ideas

Frequently Asked Questions

Why do we need a temporary next pointer?

Is linked-list reversal in-place?

Which reversal technique is best for production code?

More DSA Topic Pages

Linked List Reversal Techniques

Definition

Core Idea

Detailed Explanation

Iterative three-pointer reversal

When To Use It

Common Mistakes

Key Takeaways

Practice Ideas

Frequently Asked Questions

Why do we need a temporary next pointer?

Is linked-list reversal in-place?

Which reversal technique is best for production code?

More DSA Topic Pages