/**

* 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

*/

/**

* 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) {

if(nullptr == l1)

return l2;

if(nullptr == l2)

return l1;

ListNode *ret = l1, *pre1;

ListNode *pre2; //防止函数返回后，析构 l2 时出问题

int cf = 0;

while(l1 && l2)

{

l1->val += l2->val + cf;

cf = l1->val / 10;

l1->val %= 10;

pre1 = l1;

pre2 = l2;

l1 = l1->next;

l2 = l2->next;

}

if(nullptr == l1)

{

pre1->next = l2;

pre2->next = nullptr; //pre2的作用到此为止

l1 = l2;

}

while(l1 && cf)

{

l1->val += cf;

cf = l1->val / 10;

l1->val %= 10;

pre1 = l1;

l1 = l1->next;

}

if(cf)

{

ListNode* newNode = new ListNode(cf);

pre1->next = newNode;

}

return ret;

}

};