<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) {}
 * };


  By reversing method
 */ 
// class Solution {
// public:
// ListNode* reverseLL(ListNode* head){
//     if(!head || !head->next){
//         return head;

//     }
//     ListNode* last=reverseLL(head->next);
//     head->next->next=head;
//     head->next=NULL;
//     return last;
// }
//     ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
//         l1=reverseLL(l1);
//         l2=reverseLL(l2);
//         int sum=0;
//         int carry=0;
//         ListNode* ans= new ListNode();
//         while(l1!=NULL || l2!=NULL){
//             if(l1){
//                 sum+=l1->val;
//                 l1=l1->next;
//             }
//             if(l2){
//                 sum+=l2->val;
//                 l2=l2->next;
//             }
//             ans->val=sum%10;
//             carry=sum/10;
//             ListNode* newNode= new ListNode(carry);
//             newNode->next=ans;
//             ans=newNode;
//             sum=carry;
//         }
//         return carry==0 ? ans->next:ans;
//     }
// };

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        stack<int> s1, s2;

        while (l1 != NULL) {
            s1.push(l1->val);
            l1 = l1->next;
        }

        while (l2 != NULL) {
            s2.push(l2->val);
            l2 = l2->next;
        }

        int sum = 0, carry = 0;
        ListNode* ans = new ListNode();
        while (!s1.empty() || !s2.empty()) {
            if (!s1.empty()) {
                sum += s1.top();
                s1.pop();
            }
            if (!s2.empty()) {
                sum += s2.top();
                s2.pop();
            }

            ans->val = sum % 10;
            carry    = sum / 10;
            
            ListNode* newNode = new ListNode(carry);
            newNode->next = ans;
            ans = newNode;
            sum = carry;
        }

        return carry == 0 ? ans->next : ans;
    }
};
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