Add Two Numbers II | Problems

Given two non-empty linked lists representing two non-negative integers where the most significant digit comes first, add them and return the sum as a linked list.

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

Sample Input

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Sample Output

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Constraints

1 to 100 nodes per list
0 <= Node.val <= 9
Numbers have no leading zeros

Test Cases

Case 1

Args: [[7,2,4,3],[5,6,4]] Expected: [7,8,0,7]

Topics

Linked List

Stack-Based Addition

Use stacks to reverse the digit order, then add with carry.

function addTwoNumbersII(l1, l2) {
  let s1 = [], s2 = [];
  while (l1) { s1.push(l1.val); l1 = l1.next; }
  while (l2) { s2.push(l2.val); l2 = l2.next; }
  let carry = 0, head = null;
  while (s1.length || s2.length || carry) {
    let sum = carry + (s1.pop() || 0) + (s2.pop() || 0);
    carry = Math.floor(sum / 10);
    let node = {val: sum % 10, next: head};
    head = node;
  }
  return head;
}

Time: O(m + n) | Space: O(m + n)

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