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14. Add Two Numbers

mediumAsked at Coupang

Add two numbers stored as reverse-order linked lists, mirroring how Coupang's order-totalization pipeline streams partial sums across digit-precision boundaries during peak-event throughput.

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

Problem

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

Constraints

  • 1 <= list length <= 100
  • 0 <= node value <= 9
  • no leading zeros except the number 0

Examples

Example 1

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

Example 2

Input
l1=[9,9,9,9,9,9,9], l2=[9,9,9,9]
Output
[8,9,9,9,0,0,0,1]

Approaches

1. Convert and add

Parse both lists into BigInt, add, rebuild list.

Time
O(n)
Space
O(n)
let a = 0n, b = 0n, p = 1n;
for (let n = l1; n; n = n.next) { a += BigInt(n.val) * p; p *= 10n; }

Tradeoff:

2. Digit-by-digit with carry

Walk both lists, summing digits with a carry, and build the result list as you go.

Time
O(max(m,n))
Space
O(max(m,n))
function addTwoNumbers(l1, l2) {
  const dummy = { val: 0, next: null };
  let cur = 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);
    cur.next = { val: sum % 10, next: null };
    cur = cur.next;
    if (l1) l1 = l1.next;
    if (l2) l2 = l2.next;
  }
  return dummy.next;
}

Tradeoff:

Coupang-specific tips

Coupang's order-totalization streams partial sums across digit-precision boundaries during peak-event throughput; the carry-streaming pattern shows you understand bounded-memory arithmetic over BigInt blow-up.

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Output

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