<p>You are given two <strong>non-empty</strong> linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contains a single digit. Add the two numbers and return the sum 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/2021/04/09/sumii-linked-list.jpg" style="width: 523px; height: 342px;" />
<pre>
<strong>Input:</strong> l1 = [7,2,4,3], l2 = [5,6,4]
<strong>Output:</strong> [7,8,0,7]
</pre>

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

<pre>
<strong>Input:</strong> l1 = [2,4,3], l2 = [5,6,4]
<strong>Output:</strong> [8,0,7]
</pre>

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

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

<p>&nbsp;</p>
<p><strong>Follow up:</strong>&nbsp;Could you solve it without reversing the input lists?</p>

/**
 * 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) {
        stack<ListNode*> f;
        stack<ListNode*> s;
        ListNode* l = l1;
        ListNode* dummy = new ListNode(0, l1);
        ListNode* dummy1 = new ListNode(0, l2);
        l1 = dummy;
        l2 = dummy1;
        while (l1) {
            f.push(l1);
            l1 = l1->next;
        }
        while (l2) {
            s.push(l2);
            l2 = l2->next;
        }
        int carry = 0;
        while (!f.empty() && !s.empty()) {
            auto a = f.top();
            auto b = s.top();
            f.pop();
            s.pop();
            int n = a->val + b->val + carry;
            a->val = n%10;
            carry = n/10;
        }
        if (!s.empty()) {
            s.top()->next = dummy;
            while (!s.empty()) {
                auto a = s.top();
                cout << a->val << endl;
                s.pop();
                int n = carry + a->val;
                a->val = n%10;
                carry = n/10;
            }
            return dummy1->val ? dummy1 : dummy1->next; 
        } else {
            while (!f.empty()) {
                auto a = f.top();
                f.pop();
                int n = carry + a->val;
                a->val = n%10;
                carry = n/10;
            }
            return dummy->val ? dummy : dummy->next; 
        }
    }
};
80600