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3. Merge Two Sorted Lists

easyAsked at Postman

Merge two sorted linked lists into a single sorted list by splicing nodes.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given the heads of two sorted linked lists, merge them into one sorted linked list. The result should reuse the existing nodes from the two inputs.

Constraints

  • 0 <= list length <= 50
  • -100 <= node value <= 100
  • Both lists are sorted ascending

Examples

Example 1

Input
l1 = [1,2,4], l2 = [1,3,4]
Output
[1,1,2,3,4,4]

Example 2

Input
l1 = [], l2 = [0]
Output
[0]

Approaches

1. Brute force

Dump both lists into an array, sort, then rebuild.

Time
O((n+m) log(n+m))
Space
O(n+m)
const arr = [];
while (l1) { arr.push(l1.val); l1 = l1.next; }
while (l2) { arr.push(l2.val); l2 = l2.next; }
arr.sort((a,b)=>a-b);

Tradeoff:

2. Two-pointer splice

Walk both lists with a dummy head and append the smaller node at each step. Linear time, O(1) extra.

Time
O(n+m)
Space
O(1)
function merge(l1, l2) {
  const dummy = { next: null };
  let tail = dummy;
  while (l1 && l2) {
    if (l1.val <= l2.val) { tail.next = l1; l1 = l1.next; }
    else { tail.next = l2; l2 = l2.next; }
    tail = tail.next;
  }
  tail.next = l1 || l2;
  return dummy.next;
}

Tradeoff:

Postman-specific tips

Postman engineers value clean dummy-head patterns since merging sorted response streams is core to how the runner aggregates results from parallel HTTP requests.

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