Add Two Numbers (LeetCode)

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.

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.

Solution

public ListNode AddTwoNumbers(ListNode l1, ListNode l2) {
        ListNode output = null;
        ListNode outputCurrentNode = null;
        ListNode currentNodeL1 = l1;
        ListNode currentNodeL2 = l2;
        var carry = 0;
        var sum = 0;
        while (currentNodeL1 != null || currentNodeL2 != null) {
            if (outputCurrentNode != null) {
                outputCurrentNode.next = new ListNode(0);
                outputCurrentNode = outputCurrentNode.next;
            } else {
                outputCurrentNode = new ListNode(0);
                output = outputCurrentNode;
            }
            sum = (currentNodeL1 != null ? currentNodeL1.val : 0) + 
                    (currentNodeL2 != null ? currentNodeL2.val : 0) + 
                    carry;
            outputCurrentNode.val = sum % 10;
            carry = (sum - outputCurrentNode.val) / 10;

            if (currentNodeL1 != null) {
                currentNodeL1 = currentNodeL1.next;
            }
            if (currentNodeL2 != null) {
                currentNodeL2 = currentNodeL2.next;
            }
        }
        if (carry > 0) {
            outputCurrentNode.next = new ListNode(carry);
        }
        return output;
    }