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3. Merge Two Sorted Lists

easyAsked at Booking

Merge two sorted linked lists — Booking uses this to test pointer manipulation that mirrors merging two supplier availability streams.

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

Problem

Merge two sorted linked lists and return it as a sorted list. The new list is 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 in non-decreasing order

Examples

Example 1

Input
l1 = [1,2,4], l2 = [1,3,4]
Output
[1,1,2,3,4,4]

Example 2

Input
l1 = [], l2 = []
Output
[]

Approaches

1. Brute force array sort

Dump both lists 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);

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:

Booking-specific tips

Booking will probe whether you can extend this to merging hotel availability feeds from multiple suppliers — mention k-way merge as a follow-up.

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Output

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