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

Code:

class Solution {
public:
    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        ListNode *start= new ListNode(0);
        start->next=l1; 
        while(l1||l2){
            if(l1&&l2){
                l1->val=l1->val+l2->val;
                if(l1->val>9){
                    l1->val=l1->val%10;
                    if(l1->next)
                        l1->next->val=l1->next->val+1;
                    else if(l2->next){
                        l1->next=l2->next;
                        l2->next->val=l2->next->val+1;
                        l2=NULL;
                    }else{
                        ListNode *carry= new ListNode(1);
                        l1->next=carry;
                    }   
                }
                if(!l1->next&&l2->next){
                    l1->next=l2->next;
                    l2=NULL;
                }
                l1=l1->next;
                if(l2)
                    l2=l2->next;
            }else if(l1){
                if(l1->val>9){
                    l1->val=l1->val%10;
                    if(l1->next)
                        l1->next->val=l1->next->val+1;
                    else{
                        ListNode *carry= new ListNode(1);
                        l1->next=carry;
                    }
                }
                l1=l1->next;
            }
        }
        l1=start->next;
        delete start;
        return l1;
    }
};