Operations

Merge

Merging Two Linked Lists

Merging two linked lists means weaving their nodes together into a single ordered chain. The common technique is a two-pointer walk: look at both current heads, pick whichever is smaller, attach it to the result, and move that pointer forward.

This trick only pays off when both lists were already sorted going in — you end up with a fully sorted merged list without ever needing to run a separate sort afterward.

Tip: Merging is efficient when input lists are already sorted and doesn’t require extra space beyond a few pointers.

Steps to Merge

Create a dummy node to act as the starting point of the merged list
Use a pointer (current) to track the last node in the merged list
While both lists are non-empty, compare the current nodes
Attach the smaller node to the merged list and move that list's pointer forward
Once one list is exhausted, link the remaining part of the other list to the merged list
Return the node next to dummy as the head of the merged list

Edge Cases

Both lists are empty
One list is empty
All nodes in one list are smaller than the other
Lists have overlapping values
Input lists are not sorted (may result in unsorted merged list)

Best Practices

Ensure both input lists are sorted before merging
Always use a dummy node to simplify edge case handling
Track edge conditions where one list might be empty
Avoid modifying original input lists if immutability is required
Consider recursive merging for a concise approach (at the cost of stack space)

Visualize merging two sorted linked lists

List 1

List 2

Merged

Pointer

List 1

Generate List 1 to begin

List 2

Generate List 2 to continue

Merged List

Generate both lists and merge them

Test Your Knowledge Before Moving Forward!

Merging Quiz Challenge

How it works:

+1 point for each correct answer
0 points for wrong answers
-0.5 point penalty for viewing explanations
Earn stars based on your final score (max 5 stars)

Linked List Merging Implementation

class Node {
  constructor(data) {
    this.data = data;
    this.next = null;
  }
}

function mergeLists(l1, l2) {
  const dummy = new Node(0);
  let current = dummy;

  while (l1 && l2) {
    if (l1.data < l2.data) {
      current.next = l1;
      l1 = l1.next;
    } else {
      current.next = l2;
      l2 = l2.next;
    }
    current = current.next;
  }

  current.next = l1 || l2;
  return dummy.next;
}

Done With the Learning

Mark Merge as done and view it on your dashboard

Explore Other Operations

Insertion

Deletion

Traversal

Comparison

Searching

Reverse