Add Two Numbers

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

 /**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        if(l2==NULL) return l1;
        if(l1==NULL) return l2;
            
        struct ListNode * head=NULL;
        
        //copy l1
        struct ListNode * current=NULL;
        while(l1!=NULL)
        {
            struct ListNode * newNode=new struct ListNode(l1->val);
            if(head==NULL)
                head=newNode;
            else
                current->next=newNode;
            current=newNode;
            l1=l1->next;
        }
        //add
        struct ListNode * p1=head;
        struct ListNode * p2=l2;
        int add=0;
        while(true)
        {
            int sum;
            if(p2!=NULL)
                sum=p1->val+p2->val+add;
            else
                sum=p1->val+add;
            p1->val=sum>=10?sum-10:sum;
            add=sum>=10?1:0;
            
            if(p1->next==NULL && p2==NULL && add==0)
                break;
            if(p1->next==NULL && p2!=NULL && p2->next==NULL && add==0)
                break;
                
            if(p1->next==NULL)
                p1->next=new struct ListNode(0);
            p1=p1->next;
            if(p2!=NULL) 
                p2=p2->next;
        }
     
        return head;
    }
};