adding numbers in linked list

/**
 * 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) {
        //the obvious approach 
        //first obtain the first number by traversing l1
        //then find the second number from l2
        //add them and create a linked list with that number
        //total time O(N)+O(N)+O(N)=O(N)
        
        ListNode *headl1=l1;
        ListNode *headl2=l2;
        string templ1="";
        string templ2="";
        while(l1){
            
            templ1+=to_string(l1->val);
            l1=l1->next;
        }
        reverse(templ1.begin(),templ1.end());
          while(l2){
            
            templ2+=to_string(l2->val);
            l2=l2->next;
        }
        reverse(templ2.begin(),templ2.end());
        int no1=stoi(templ1);
        int no2=stoi(templ2);
        int result=no1+no2;
  ListNode *prev=NULL;
        ListNode *head=NULL;
        if(result==0){
            return new ListNode(0);
        }
        
        while(result){
            int remainder=result%10;
            result=result/10;
            ListNode *newnode=new ListNode(remainder);
            if(prev==NULL)
                head=newnode;
            else
                prev->next=newnode;
            prev=newnode;
        }
        return head;
    }
};