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14. Add Two Numbers

mediumAsked at Grab

Add two linked-list digits with carry — Grab uses this to test pointer + arithmetic handling in one shot.

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

Problem

You are given two non-empty linked lists representing two non-negative integers, with digits stored in reverse order. Add the numbers and return the sum as a linked list.

Constraints

  • 1 <= length of each list <= 100
  • 0 <= Node.val <= 9
  • Numbers have no leading zeros except 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 integers

Read both lists into BigInt, add, then build the result list.

Time
O(n)
Space
O(n)
// fails on lists longer than 64 digits without BigInt
// brittle for partition shards across SEA regions

Tradeoff:

2. Single-pass with carry

Walk both lists simultaneously, sum each digit pair plus carry, append a node per digit.

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 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:

Grab-specific tips

Grab interviewers reward candidates who handle the carry loop cleanly — frame as adding partitioned wallet balances across regional shards.

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Output

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