<p>You are given two <strong>non-empty</strong> linked lists representing two non-negative integers. The digits are stored in <strong>reverse order</strong>, and each of their nodes contains a single digit. Add the two numbers and return the sum&nbsp;as a linked list.</p>

<p>You may assume the two numbers do not contain any leading zero, except the number 0 itself.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/10/02/addtwonumber1.jpg" style="width: 483px; height: 342px;" />
<pre>
<strong>Input:</strong> l1 = [2,4,3], l2 = [5,6,4]
<strong>Output:</strong> [7,0,8]
<strong>Explanation:</strong> 342 + 465 = 807.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> l1 = [0], l2 = [0]
<strong>Output:</strong> [0]
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]
<strong>Output:</strong> [8,9,9,9,0,0,0,1]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in each linked list is in the range <code>[1, 100]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 9</code></li>
	<li>It is guaranteed that the list represents a number that does not have leading zeros.</li>
</ul>

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        // ListNode* total;
        // bool carry;
        // if ((l1->val + l2->val) < 10){
        //     total = new ListNode(l1->val + l2->val);
        //     carry = false;
        // } else {
        //     total = new ListNode(l1->val + l2->val) - 10;
        //     carry = true;
        // }
        // l1 = l1->next;
        // l2 = l2->next;
        // ListNode* current = total;
        
        // while (l1 && l2) {
        //     int sum = l1->val + l2->val + carry;
        //     if (sum < 10) {
        //         current->next = new ListNode(sum);
        //         carry = false;
        //     } else {
        //         current->next = new ListNode(sum-10);
        //         carry = true;
        //     }
        //     current = current->next;
        //     l1 = l1->next;
        //     l2 = l2->next;
        // }

        // if (l1) {
        //     l1->val += carry;
        //     current->next = l1;
        // } else if (l2) {
        //     l2->val += carry;
        //     current->next = l2;
        // }

        // return total;

        /* start with a dummy Node*/
        ListNode* total = new ListNode(0);
        ListNode* current = total;
        int carry = 0;

        while (l1 || l2 || carry) {
            int sum = carry; 
            if (l1) {
                sum += l1 -> val;
                l1 = l1 -> next;
            } 
            if (l2) {
                sum += l2 -> val;
                l2 = l2 -> next;
            }

            carry  = sum / 10;
            sum = sum % 10;

            current->next = new ListNode(sum);
            current = current-> next;
        }
        return total->next;
    }
};
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