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3. Merge Two Sorted Lists

easyAsked at Klarna

Merge two sorted linked lists into one sorted list by splicing nodes.

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

Problem

You are given the heads of two sorted linked lists list1 and list2. Merge the two lists into one sorted list by splicing together the nodes of the first two lists and return the head.

Constraints

  • 0 <= list length <= 50
  • -100 <= Node.val <= 100
  • Both lists are sorted in non-decreasing order.

Examples

Example 1

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

Example 2

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

Approaches

1. Collect and sort

Push all values into an array, sort, rebuild list.

Time
O(n log n)
Space
O(n)
const vals = [];
while (list1) { vals.push(list1.val); list1 = list1.next; }
while (list2) { vals.push(list2.val); list2 = list2.next; }
vals.sort((a,b)=>a-b);
// rebuild list

Tradeoff:

2. Two-pointer splice

Walk both lists, attaching smaller node to tail. Single pass, no extra allocation.

Time
O(n+m)
Space
O(1)
function mergeTwoLists(l1, l2) {
  const dummy = { val: 0, 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:

Klarna-specific tips

Klarna values in-place pointer manipulation since their settlement reconciliation pipelines merge sorted streams of installment events without allocating per-record.

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