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3. Merge Two Sorted Lists

easyAsked at Dropbox

Merge two sorted linked lists into one sorted list; Dropbox uses it as a stand-in for merging two sorted delta streams from different clients.

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

Problem

You are given the heads of two sorted linked lists. Merge them into a single sorted linked list by splicing nodes (not copying values). Return the head.

Constraints

  • 0 <= n <= 50 nodes per list
  • -100 <= Node.val <= 100
  • Both inputs are 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. Brute force

Collect both 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}
return arr.sort((a,b)=>a-b).reduce(...);

Tradeoff:

2. Two-pointer splice

Use a dummy head and advance the pointer with the smaller value at each step. No extra allocation.

Time
O(n + m)
Space
O(1)
function merge(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:

Dropbox-specific tips

Dropbox favors in-place splicing — they map allocation cost to memory pressure on the sync daemon and dock candidates who copy nodes.

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Output

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