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3. Merge Two Sorted Lists

easyAsked at Monzo

Merge two sorted linked lists of transactions 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 a single sorted list by splicing together the nodes. Return the head of the merged list.

Constraints

  • 0 <= length of each list <= 50
  • Lists are 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

Collect all values into array, sort, rebuild list.

Time
O((n+m) log(n+m))
Space
O(n+m)
// Ignores the sorted property — wasteful.

Tradeoff:

2. Two-pointer splice

Walk both lists with a dummy head, 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:

Monzo-specific tips

Monzo likes seeing the dummy-head pattern — it signals you've worked with real production list code, not just toy problems.

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Output

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