<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* head = l1;
        ListNode* head2 = l2;
        int size1=0;
        int size2=0;
        while(l1 || l2){
            if(l1) {
                stk1.push(l1);
                l1 = l1->next;
                ++size1;
            }
            if(l2){
                stk2.push(l2);
                l2 = l2->next;
                ++size2;
            }
        }
        if(size2>size1) head = head2;
        int re = 0;
        if(size2>size1){
            while(!stk2.empty()){
                int a = 0;
                if(!stk1.empty()){ a = stk1.top()->val; stk1.pop();}
                int b = stk2.top()->val;

                stk2.top()->val = (re+a+b)%10;
                re = (re+a+b)/10;
                stk2.pop();
            }
        }
        else{
            while(!stk1.empty()){
                int a = 0;
                if(!stk2.empty()){ a = stk2.top()->val; stk2.pop();}
                int b = stk1.top()->val;

                stk1.top()->val = (re+a+b)%10;
                re = (re+a+b)/10;
                stk1.pop();
            }
        }
        if(!re){
            return head;
        }
        ListNode* r = new ListNode(re);
        r->next = head;
        return r;


    }
};
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