2. Add Two Numbers

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}   初始化支持 ListNode（int x）这种方式，即 把x赋给val，next赋值NULL。
* };
*/
class Solution {
public:
ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
ListNode *head = new ListNode(0);
ListNode *p = head;
int tag = 0 ;
while(true){
if(l1 != NULL){
tag+=l1->val;
l1 = l1->next;
}
if(l2 != NULL){
tag+=l2->val;
l2 = l2->next;
}
p->val = tag%10;
tag = tag/10;
if(l1 != NULL || l2 != NULL || tag != 0){
p = (p->next = new  ListNode(0));
}
else break;
}
return head;
}
};