Leetcode2. Add Two Numbers

/**
 * 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) {
        int extra=0,sum=0;
        ListNode start(-1);//设置头结点
        ListNode* p=&start;//设置指针   作用是指针可以直接指到下一个指针p=p->next;
        while(l1!=nullptr||l2!=nullptr){
            int val1=l1==nullptr?0:l1.val;
            int val2=l2==nullptr?0:l2.val;
            int sum=val1+val2+extra;
            extra=sum/10;
            p->next=new ListNode(sum%10);
            p=p->next;
            l1=l1==nullptr?l1:l1->next;
            l2=l2==nullptr?l2:l2->next;
        }
        return start->next;
    }
};