<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<int>st1;
        stack<int>st2;

        ListNode * temp = l1;

        while(temp != NULL){
            st1.push(temp->val);
            temp = temp->next;
        }

        temp = l2;

        while(temp != NULL){
            st2.push(temp->val);
            temp = temp->next;
        }

        int firstvalue = 0;
        int carry = 0;

        if(!st1.empty() && !st2.empty()){
            int num = st1.top() + st2.top();
            if(num > 9){
                int rem = num % 10;
                carry = num / 10;
                firstvalue = rem;
            }
            else{
                firstvalue = num;
            }
            st1.pop();
            st2.pop();
        }

        ListNode * newList = new ListNode(firstvalue);
        ListNode * mover = newList;

        while(!st1.empty() && !st2.empty()){
            int num = st1.top() + st2.top() + carry;
            if(num > 9){
                int rem = num % 10;
                carry = num / 10;
                firstvalue = rem;
            }
            else{
                firstvalue = num;
                carry = 0;
            }

            ListNode * newNode = new ListNode(firstvalue);
            mover->next = newNode;
            mover = newNode;
            st1.pop();
            st2.pop();
        }

        while(!st1.empty()){
            int num = st1.top() + carry;
            if(num > 9){
                int rem = num % 10;
                int carry = num / 10;
                firstvalue = rem;
            }
            else{
                firstvalue = num;
                carry = 0;
            }
            ListNode * newNode = new ListNode(firstvalue);
            mover->next = newNode;
            mover = newNode;
            st1.pop();
        }

        while(!st2.empty()){
            int num = st2.top() + carry;
            if(num > 9){
                int rem = num % 10;
                int carry = num / 10;
                firstvalue = rem;
            }
            else{
                firstvalue = num;
                carry = 0;
            }
            ListNode * newNode = new ListNode(firstvalue);
            mover->next = newNode;
            mover = newNode;
            st2.pop();
        }

        if(carry > 0){
            ListNode * newNode = new ListNode(carry);
            mover->next = newNode;
            mover = newNode;
        }

        ListNode * next = newList;
        ListNode * prev = nullptr;
        temp = newList;

        while(temp != NULL){
            next = temp->next;
            temp->next = prev;
            prev = temp;
            temp = next;
        }

        return prev;

        
    }
};
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