<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* insertAtHead(ListNode* head, ListNode* newHead){
        if(head){
            newHead -> next = head;
        }       
        return newHead;
    }

    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {

        ListNode* temp = l1;

        stack<int> s1;

        while(temp){
            s1.push(temp->val);
            temp = temp -> next;
        }

        temp = l2;

        stack<int> s2;

        while(temp){
            s2.push(temp->val);
            temp = temp -> next;
        }

        ListNode* head = NULL;

        int carry = 0;

        while(!s1.empty() || !s2.empty() || carry){
            
            int sum = carry;
            
            if(!s1.empty()){
                sum += s1.top();
                s1.pop();
            }
            if(!s2.empty()){
                sum += s2.top();
                s2.pop();
            }

            carry = sum/10;

            head  = insertAtHead(head, new ListNode(sum%10));
        }

        return head;
    }
};
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