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3. Merge Two Sorted Lists

easyAsked at Spotify

Merge two sorted 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 them into one sorted linked list and return the head.

Constraints

  • 0 <= nodes <= 50
  • -100 <= Node.val <= 100
  • Both lists are sorted ascending

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. Brute force

Collect into array, 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 from vals

Tradeoff:

2. Iterative two-pointer

Maintain a dummy head and append smaller node each step. Skips re-sorting and runs in linear time.

Time
O(n+m)
Space
O(1)
function mergeTwoLists(l1, l2) {
  const dummy = { val: 0, 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:

Spotify-specific tips

Spotify cares about clean pointer hygiene because playlist-segment merges and rolling audio-buffer joins use the same primitive — narrate edge cases (empty list, equal vals) out loud.

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