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3. Merge Two Sorted Lists

easyAsked at MercadoLibre

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 linked lists list1 and list2. Merge them into one sorted list by splicing together the nodes of the first two lists. Return the head of the merged list.

Constraints

  • 0 <= list length <= 50
  • -100 <= node value <= 100
  • Both lists are 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. Brute force

Collect values to an array, sort, rebuild list.

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

Walk both lists with a dummy head, picking the smaller node each step.

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:

MercadoLibre-specific tips

MercadoLibre uses this to confirm dummy-node and pointer hygiene, which matter for their search-result merge pipelines combining multiple ranked sources.

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Output

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