<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) {
        if(l1 == NULL) return l2;
        if(l2 == NULL) return l1;

        int carry = 0;
        // the solution will return the linked list l1 as the head
        ListNode* ans = l1;
        ListNode* prev;
        while(l1 != NULL && l2 != NULL){
            int sum = l1 -> val + l2 -> val + carry;
            carry = sum / 10;
            sum = sum % 10;
            l1 -> val = sum;
            prev = l1;
            l1 = l1 -> next;
            l2 = l2 -> next;
        }
        while(l1 != NULL){
            int sum = l1 -> val + carry;
            carry = sum / 10;
            sum = sum % 10;
            l1 -> val = sum;
            prev = l1;
            l1 = l1 -> next;
        }
        while(l2 != NULL){
            int sum = l2 -> val + carry;
            carry = sum / 10;
            sum = sum % 10;
            l2 -> val = sum;
            prev -> next = l2;
            prev = l2;
            l2 = l2 -> next;
        }
        if(carry == 1){
            ListNode* lastNode = new ListNode(1);
            prev -> next = lastNode;
        }
        return ans;
    }
};
76800