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12. Add Two Numbers

mediumAsked at Klarna

Add two non-negative integers represented as reversed linked lists, digit per node.

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

Problem

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each node contains a single digit. Add the two numbers and return the sum as a linked list.

Constraints

  • 1 <= node count <= 100
  • 0 <= Node.val <= 9
  • No leading zeros except the number 0 itself

Examples

Example 1

Input
l1 = [2,4,3], l2 = [5,6,4]
Output
[7,0,8]

Example 2

Input
l1 = [0], l2 = [0]
Output
[0]

Approaches

1. Convert to BigInt, add, rebuild

Read both lists into BigInt, add, then write digits back out.

Time
O(n)
Space
O(n)
function addTwoNumbers(l1, l2) {
  const toBig = (l) => { let s = '', c = l; while (c) { s = c.val + s; c = c.next; } return BigInt(s); };
  let sum = (toBig(l1) + toBig(l2)).toString();
  const dummy = { next: null };
  let tail = dummy;
  for (const d of sum) { tail = tail.next = { val: +d, next: null }; }
  // reverse...
  return dummy.next;
}

Tradeoff:

2. Single-pass digit-by-digit add

Walk both lists together, tracking a carry digit. The reversed encoding means least-significant digit is first, so addition flows naturally without reversing anything.

Time
O(max(m,n))
Space
O(max(m,n))
function addTwoNumbers(l1, l2) {
  const dummy = { val: 0, next: null };
  let tail = dummy, carry = 0;
  while (l1 || l2 || carry) {
    const a = l1 ? l1.val : 0;
    const b = l2 ? l2.val : 0;
    const sum = a + b + carry;
    carry = Math.floor(sum / 10);
    tail.next = { val: sum % 10, next: null };
    tail = tail.next;
    if (l1) l1 = l1.next;
    if (l2) l2 = l2.next;
  }
  return dummy.next;
}

Tradeoff:

Klarna-specific tips

Klarna treats this as a stand-in for cross-currency installment-ledger arithmetic; they will probe whether you handle the trailing carry and never silently truncate cents.

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