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3. Merge Two Sorted Lists

easyAsked at SoFi

Merge two sorted linked lists — SoFi maps this to merging two sorted streams of loan-payment events.

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

Problem

You are given the heads of two sorted singly linked lists. Return a single sorted list spliced from the two inputs.

Constraints

  • 0 <= n1, n2 <= 50
  • -100 <= Node.val <= 100
  • Both lists are sorted ascending

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

Collect all values, sort, build a new list.

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);
// build linked list from arr

Tradeoff:

2. Two-pointer merge

Walk both lists, splicing the smaller node onto a dummy tail. Linear time and the cleaner answer SoFi expects.

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:

SoFi-specific tips

SoFi expects clean dummy-head technique and a verbal note that the two-pointer merge mirrors how their payment-reconciliation service joins ACH and card streams.

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