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31. Add Two Numbers

mediumAsked at Snowflake

Add two non-negative integers stored as linked lists in reverse-digit order. Snowflake asks this to test carry propagation logic — the same arithmetic kernel that underlies their high-precision NUMBER type's addition.

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

Source citations

Public interview reports confirming this problem appears in Snowflake loops.

  • Glassdoor (2026-Q1)— Snowflake numerics-team uses this as canonical big-number arithmetic warm-up.
  • LeetCode Discuss (2025-11)— Recurring at Snowflake SDE-I onsites.

Problem

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list. You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Constraints

  • The number of nodes in each linked list is in the range [1, 100].
  • 0 <= Node.val <= 9
  • It is guaranteed that the list represents a number that does not have leading zeros.

Examples

Example 1

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

Explanation: 342 + 465 = 807.

Example 2

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

Example 3

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 to numbers, add, rebuild

Walk both lists to build BigInts, add, walk back to a list.

Time
O(max(m, n))
Space
O(max(m, n))
function addTwoNumbers(l1, l2) {
  function toBig(node) {
    let n = 0n, place = 1n;
    while (node) { n += BigInt(node.val) * place; place *= 10n; node = node.next; }
    return n;
  }
  let sum = toBig(l1) + toBig(l2);
  const dummy = { val: 0, next: null };
  let tail = dummy;
  if (sum === 0n) return { val: 0, next: null };
  while (sum > 0n) {
    tail.next = { val: Number(sum % 10n), next: null };
    tail = tail.next;
    sum /= 10n;
  }
  return dummy.next;
}

Tradeoff: Cute thanks to BigInt, but misses the point of the problem — show carry propagation.

2. Walk both lists with carry (optimal)

Walk both lists in parallel. At each step: sum = l1.val + l2.val + carry; new digit = sum % 10; carry = sum / 10. Stop when both null and carry == 0.

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

Tradeoff: Streams digit-by-digit, exactly how a fixed-point arithmetic kernel works. Handles unequal lengths and final carry uniformly.

Snowflake-specific tips

Snowflake interviewers want carry handled in the loop condition, not as a special case afterward. Bonus signal: connect to NUMBER(38,2) addition — Snowflake's fixed-point arithmetic uses digit-by-digit addition with carry on the underlying decimal limbs.

Common mistakes

  • Forgetting the final carry — list [5] + [5] = [0, 1], not [0].
  • Skipping the unequal-length case by stopping when one list ends.
  • Trying parseInt on long lists — overflows beyond 15 digits with regular Number.

Follow-up questions

An interviewer at Snowflake may pivot to one of these next:

  • Add Two Numbers II (LC 445) — digits stored in forward order.
  • Multiply Strings (LC 43).
  • How does Snowflake's NUMBER(38) addition work?

Solve it now

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Output

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FAQ

Why include carry in the loop condition?

Otherwise [9] + [9] = [8] — you miss the final carry. The condition (l1 || l2 || carry) unifies all three cases.

Why reverse order?

Reverse-order storage means digits are ordered LSB-first, which is the natural direction for carry propagation. Forward order requires a stack or recursion.

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