题目描述：
You are given two non-empty linked lists representing two non-negative integers. 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.
You may assume the two numbers do not contain any leading zero, except the number 0 itself.
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
思路：
- 两个链表长度可能不同；
- 最后一个节点相加后可能产生进位。
代码实现：
/**
* 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) {
ListNode* node1 = l1;
ListNode* node2 = l2;
ListNode* Node = new ListNode(0);
ListNode* p = Node;
int num = 0;
while(node1!=NULL || node2!=NULL){
if(node1!=NULL){
num += node1->val;
node1 = node1->next;
}
if(node2!=NULL){
num += node2->val;
node2 = node2->next;
}
p->next = new ListNode(num%10);
p = p->next;
num /= 10;
}
if(num==1){
p->next = new ListNode(1);
}
return Node->next;
}
};