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3. Merge Two Sorted Lists

easyAsked at Slack

Merge two sorted singly linked lists 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 the two lists into one sorted list. The list should be made by splicing together the nodes of the first two lists.

Constraints

  • 0 <= each list length <= 50
  • -100 <= Node.val <= 100
  • Both 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

Copy all values into an array, sort, rebuild.

Time
O((n+m) log(n+m))
Space
O(n+m)
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

Tradeoff:

2. Two-pointer splice

Use a dummy head and a tail pointer. Pick the smaller head each step. Append leftover.

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:

Slack-specific tips

Slack's message-history merge (across reconnects) is a real-world variant — mention how you'd merge two ordered message streams by ts.

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