runtime error: member access within null pointer of type 'struct ListNode' ，该怎么改？求帮助！

Question

leetcode 第二题Add Two Numbers，我的回答出现runtime error: member access within null pointer of type 'struct ListNode' ，该怎么改？求帮助！

问题：把两个链表各位相加，得到一个新链表
Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

 class Solution {
public:
    //把链表转化成整数
    int ConverseToInt(ListNode* l){
            int num=0,mul=1;
            while(l!=NULL){
                num=num+mul*l->val;
                l=l->next;
                mul*=10;
            }
            return num;
        }
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {

        int a=ConverseToInt(l1);
        int b=ConverseToInt(l2);

        int c=a+b;
        ListNode *newNumber=NULL;
        newNumber->val=c%10;
        c/=10;
        ListNode* returnNumber=NULL;
        returnNumber=newNumber;
        newNumber=newNumber->next;
        while(c!=0){
            newNumber->val=c%10;
            c/=10;
            newNumber=newNumber->next;
        }
        return returnNumber;
    }
};  

Answer 1

ListNode *newNumber=NULL;

newNumber->val=c%10;

这句代码错了，你的newNumber是空指针。