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.

/**
 * 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) {
        int digit=0,carry=0;        
        ListNode* answer=new ListNode(0),*p=answer;
        while(l1 || l2 || carry){
            digit=carry;
            if(l1){
                digit+=l1->val;
                l1=l1->next;
            }
            if(l2){
                digit+=l2->val;                
                l2=l2->next;
            }            
            carry=digit/10;
            if(digit>9)
                digit%=10;
            p->next=new ListNode(digit);
            p=p->next;
        }        
        return answer->next;
    }
};