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3. Merge Two Sorted Lists

easyAsked at Squarespace

Merge two sorted linked lists into one; Squarespace uses it to verify pointer hygiene with dummy heads.

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

Problem

Given heads of two sorted linked lists, splice them together into a single sorted list and return the head.

Constraints

  • 0 <= total nodes <= 50
  • -100 <= Node.val <= 100

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 all values to an array, sort, rebuild.

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

Tradeoff:

2. Dummy-head merge

Walk both pointers, attach 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:

Squarespace-specific tips

Squarespace expects a dummy-head and an explicit comment on why it cleans up edge cases when stitching nodes — same hygiene applies to merging two ordered block streams in their layout engine.

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Output

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