<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*> stk1, stk2;
        ListNode* ptr1 = l1, *ptr2 = l2;

        while(ptr1 || ptr2)
        {
            if(ptr1)
            {
                stk1.push(ptr1);   ptr1 = ptr1->next;
            }
            if(ptr2)
            {
                stk2.push(ptr2);   ptr2 = ptr2->next;
            }
        }
    
        int carry = 0;
        while(stk1.empty() == false && stk2.empty() == false)
        {
            ptr1 = stk1.top();   ptr2 = stk2.top();
            int res = ptr1->val + ptr2->val + carry;
            stk1.pop();    stk2.pop();

            if(res>9)
            {
                res = res%10;
                carry = 1;
            }
            else
            {
                carry = 0;
            }
            ptr1->val = res;
            ptr2->val = res;
        }

        stack<ListNode*> *stk = stk1.empty() == true ? &stk2 : &stk1;
        while(stk->empty() == false)
        {
            ptr1 = stk->top();
            int res = ptr1->val + carry;
            if(res > 9)
            {
                res = res%10;
                carry = 1;
            }
            else
            {
                carry = 0;
            }

            ptr1->val = res;
            stk->pop();
        }

        if(carry == 1)
        {
            ListNode* node = new ListNode(carry);
            node->next = ptr1;
            ptr1 = node;
        }

        return ptr1;
    }
};
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