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3. Merge Two Sorted Lists

easyAsked at Etsy

Merge two sorted linked lists into one sorted list — Etsy uses this to gauge pointer hygiene and edge-case discipline.

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

Problem

You are given the heads of two sorted linked lists list1 and list2. Splice them together into one sorted list and return the head.

Constraints

  • 0 <= n1 + n2 <= 5000
  • -100 <= Node.val <= 100
  • Both inputs sorted ascending

Examples

Example 1

Input
list1 = [1,2,4], list2 = [1,3,4]
Output
[1,1,2,3,4,4]

Example 2

Input
list1 = [], list2 = [0]
Output
[0]

Approaches

1. Collect and sort

Dump both lists into an array, sort, rebuild.

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

Tradeoff:

2. Two-pointer splice

Use a dummy node and walk both lists, attaching the smaller head each step.

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:

Etsy-specific tips

Etsy interviewers care about how you'd merge two pre-sorted listing streams (e.g. ML-ranked vs. recency) without re-sorting — frame the pointer answer in those terms.

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