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3. Merge Two Sorted Lists

easyAsked at Duolingo

Merge two sorted linked lists into one sorted list.

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

Problem

You are given the heads of two sorted singly linked lists. Splice them together into a single sorted linked list and return the new head. The new list should be made by reusing the nodes of the two input lists.

Constraints

  • 0 <= each list length <= 50
  • -100 <= Node.val <= 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. Collect and sort

Push every value into an array, sort, and rebuild a list.

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

Tradeoff:

2. Two-pointer splice

Walk both lists with two pointers, attach the smaller head, advance, and append the leftover tail.

Time
O(n+m)
Space
O(1)
function mergeTwoLists(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:

Duolingo-specific tips

Duolingo learners progress through ordered skill queues; talk about how this is the core merge step inside a spaced-repetition review scheduler.

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