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3. Merge Two Sorted Lists

easyAsked at TripAdvisor

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

Constraints

  • 0 <= length <= 50
  • -100 <= Node.val <= 100
  • Both lists are sorted in non-decreasing order

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

Extract all values, 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);
// rebuild list

Tradeoff:

2. Two-pointer splice

Walk both lists, attach smaller node to tail. Reuses existing nodes.

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:

TripAdvisor-specific tips

TripAdvisor uses this pattern when merging review feeds from multiple sources into a single sorted timeline.

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