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14. Add Two Numbers

mediumAsked at MercadoLibre

Sum two integers represented as reverse-order linked lists, digit by digit.

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

Problem

Two non-empty linked lists represent two non-negative integers with digits stored in reverse order. Add the two numbers and return the sum as a linked list in the same reverse order.

Constraints

  • 1 <= node count <= 100
  • 0 <= Node.val <= 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], l2 = [1]
Output
[0,0,0,1]

Approaches

1. Convert to bigints

Read both lists into BigInts, sum, then rebuild a list. Fragile on huge sizes and overkill.

Time
O(n)
Space
O(n)
const toNum = (l) => { let s = '', n = l; while (n) { s = n.val + s; n = n.next; } return BigInt(s); };
const sum = toNum(l1) + toNum(l2);
// rebuild list from sum.toString()...

Tradeoff:

2. Digit-wise with carry

Walk both lists in lockstep, sum digits with a carry, and append each new digit to a result list.

Time
O(max(m,n))
Space
O(max(m,n))
function addTwoNumbers(l1, l2) {
  const dummy = { next: null };
  let cur = dummy, carry = 0;
  while (l1 || l2 || carry) {
    const v = (l1?.val || 0) + (l2?.val || 0) + carry;
    carry = (v / 10) | 0;
    cur.next = { val: v % 10, next: null };
    cur = cur.next;
    l1 = l1?.next; l2 = l2?.next;
  }
  return dummy.next;
}

Tradeoff:

MercadoLibre-specific tips

Mercado Pago ledger engineers want clean carry handling because their multi-currency settlement service adds digits across millions of low-value pesos and centavos transactions every minute.

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Output

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