/**
 * 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) {
        ListNode* res=new ListNode();
        ListNode* ref=res;
        stack<int> n1;
        stack<int> n2;
        stack<int> ans;
        int c=0;
        
        while(l1 || l2)
        {
            if(l1!=nullptr)
            {
                n1.push(l1->val);
                l1=l1->next;
            }
            if(l2!=nullptr)
            {
                n2.push(l2->val);
                l2=l2->next;
            }
        }
        
        while(!n1.empty() || !n2.empty() || c)
        {
            if(!n1.empty())
            {
                c+=n1.top();
                n1.pop();
            }
            if(!n2.empty())
            {
                c+=n2.top();
                n2.pop();
            }
            
            ans.push(c%10);
            c/=10;
        }
        while(!ans.empty())
        {
            ref->next = new ListNode(ans.top());
            ans.pop();
            ref=ref->next;
        }
        
        return res->next;
    }
};