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3. Merge Two Sorted Lists

easyAsked at Ramp

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

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

Problem

Given the heads of two sorted singly linked lists, splice them into a single sorted list and return its head. Reuse the existing nodes; do not allocate new ones.

Constraints

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

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 into array, sort, rebuild 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

Tradeoff:

2. Two-pointer splice

Walk both pointers, attach smaller node to tail, advance. Linear time, no extra allocation.

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:

Ramp-specific tips

Ramp often reframes this as merging two sorted streams of card transactions while preserving stable ordering for ledger postings, so emphasize O(1) extra space and stable tie-breaks.

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