<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* head1, ListNode* head2) {
        
        stack<int> st1, st2;
    
    ListNode* temp1 = head1;
    while (temp1 != nullptr) {
        st1.push(temp1->val);
        temp1 = temp1->next;
    }

    ListNode* temp2 = head2;
    while (temp2 != nullptr) {
        st2.push(temp2->val);
        temp2 = temp2->next;
    }

    vector<int> v1;
    int carry = 0;

    while (!st1.empty() || !st2.empty()) {
        int sum = carry;
        if (!st1.empty()) {
            sum += st1.top();
            st1.pop();
        }
        if (!st2.empty()) {
            sum += st2.top();
            st2.pop();
        }
        v1.push_back(sum % 10);
        carry = sum / 10;
    }

    if (carry) {
        v1.push_back(carry);
    }

    ListNode* head3 = new ListNode(v1.back());
    ListNode* temp3 = head3;

    for (int i = v1.size() - 2; i >= 0; i--) {
        temp3->next = new ListNode(v1[i]);
        temp3 = temp3->next;
    }

    return head3;
    }
};
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