2-add-two-numbers.ts

/** 
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.

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]
 
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.
*/

class ListNode {
  val: number;
  next: ListNode | null;
  constructor(val?: number, next?: ListNode | null) {
    this.val = val === undefined ? 0 : val;
    this.next = next === undefined ? null : next;
  }
}

function addTwoNumbers(
  l1: ListNode | null,
  l2: ListNode | null
): ListNode | null {
  let current = new ListNode(0);
  let result = current;

  let carry = 0;

  while (l1 || l2 || carry) {
    let value1 = l1 ? l1.val : 0;
    let value2 = l2 ? l2.val : 0;

    let sum = value1 + value2 + carry;

    carry = 0;

    if (sum > 9) {
      carry = ~~(sum / 10);
      sum = sum % 10;
    }

    current.next = new ListNode(sum);
    current = current.next;

    if (l1) l1 = l1.next;
    if (l2) l2 = l2.next;
  }

  return result.next;
}