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3. Merge Two Sorted Lists

easyAsked at CircleCI

Splice two sorted linked lists into a single 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 the two lists into a single sorted list by splicing nodes and return the head of the merged list.

Constraints

  • 0 <= list length <= 50
  • -100 <= Node.val <= 100
  • Both inputs sorted non-decreasing

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

Pull all values into an array, sort, then rebuild a list.

Time
O(n log n)
Space
O(n)
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);
// rebuild list from arr...

Tradeoff:

2. Two-pointer splice

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

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:

CircleCI-specific tips

CircleCI engineers ship merging logic constantly (job results, log streams) so they prize the in-place splice over array rebuild.

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Output

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