LeetCode Add Two Numbers

You are given two linked lists representing two non-negative numbers. 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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

模拟加法，注意最后加法结束时可能有进位需要处理一下。

/**
* 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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if (l1 == NULL && l2 == NULL)
return NULL;

ListNode *head = NULL;
ListNode *pre = NULL;
int flag = 0;

while(l1 && l2)
{
ListNode *node = new ListNode(l1->val + l2->val + flag);
flag = node->val / 10;
node->val %= 10;

if (head == NULL)
head = node;
else
pre->next = node;

pre = node;

l1 = l1->next;
l2 = l2->next;
}

while(l1)
{
ListNode *node = new ListNode(l1->val + flag);
flag = node->val / 10;
node->val %= 10;

if (head == NULL)
head = node;
else
pre->next = node;

pre = node;

l1 = l1->next;
}

while(l2)
{
ListNode *node = new ListNode(l2->val + flag);
flag = node->val / 10;
node->val %= 10;

if (head == NULL)
head = node;
else
pre->next = node;

pre = node;

l2 = l2->next;
}

if (flag > 0)
{
ListNode *node = new ListNode(flag);
pre->next = node;
}

return head;
}
};