## Problem

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.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.


Leetcode: English, Chinese

## Solution

Just simulate. Use a dummy node to avoid extra conditional statements for initializing the head’s value.

## Code

/**
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/

class Solution {
public:
ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
ListNode* dummy = new ListNode(0);
ListNode* p = dummy;
int carry = 0;

while (l1 || l2 || carry) {
int x = l1 ? l1->val : 0;
int y = l2 ? l2->val : 0;

int sum = x + y + carry;
carry = sum / 10;

p->next = new ListNode(sum % 10);
p = p->next;

if (l1) l1 = l1->next;
if (l2) l2 = l2->next;
}
return dummy->next;
}
};