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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        //case 1: l1 or l2 is empty
        if (l1==NULL) return l2;
        if (l2==NULL) return l1;
    
        //normal case
        ListNode* newHead = NULL;
        ListNode* prep = NULL;
        ListNode* curr = NULL;
        int sum;
        int reminder;
        bool plus;
       
        newHead = new ListNode(l1->val+l2->val);
        if(newHead->val >= 10){
        newHead->val = newHead->val - 10;
        plus = true;
        }
        else{
        plus = false;
        }
        prep = newHead;
        l1=l1->next;
        l2=l2->next;
       
        while((l1!=NULL)&&(l2!=NULL)){
         sum = l1->val+l2->val;
         if(plus) sum++;
        
         if(sum>=10){
         sum = sum - 10;
         plus = true;
         }
         else {
         plus = false;
         }
        
         curr = new ListNode(sum);
         prep->next = curr;        
         prep = curr;
         l1=l1->next;
         l2=l2->next;
        }
       
        curr = NULL;
       
        if(l1!=NULL){
        curr = l1;
        }
       
        if(l2!=NULL){
        curr = l2;
        }
       
        if((l1==NULL)&&(l2==NULL)&&plus){
        curr = new ListNode(1);
        plus = false;
        }
       
        while(curr!=NULL){
        if(plus) curr->val++;
        if(curr->val >= 10){
        curr->val = curr->val-10;
        plus = true;
        }
        else{
        plus = false;
        }
        prep->next = curr;
        prep = curr;
        curr = curr->next;
        }
       
        if(plus){
        curr = new ListNode(1);
        prep->next = curr;
        }
       
        return newHead;
    }
};