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3. Merge Two Sorted Lists

easyAsked at Expedia

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 in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

Constraints

  • 0 <= count of nodes <= 50
  • -100 <= Node.val <= 100
  • Both lists 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 = []
Output
[]

Approaches

1. Collect and sort

Dump 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; }
arr.sort((a,b)=>a-b);

Tradeoff:

2. Two-pointer merge

Walk both lists, append smaller node to result. Expedia uses this exact technique to merge sorted hotel-rate feeds from competing suppliers.

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:

Expedia-specific tips

Expedia interviewers will probe how you'd extend this to k sorted streams — a direct analog to merging price feeds from k supplier APIs.

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