Question：

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

Anwser 1：

/**
 * 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
        if(l1 == NULL && l2 == NULL) {
            return NULL;
        }
        
        ListNode *retList = new ListNode(0);
        ListNode *tmpList = retList;
        
        ListNode *head1 = l1;
        ListNode *head2 = l2;
        int carry = 0;
        while(head1 != NULL && head2 != NULL){
            int sum = carry + head1->val + head2->val;
            carry = sum / 10;
            tmpList->next = new ListNode(sum % 10);
            tmpList = tmpList->next;
            
            head1 = head1->next;
            head2 = head2->next;
        }
        
        while(head1 != NULL){
            int sum = carry + head1->val;
            carry = sum / 10;
            tmpList->next = new ListNode(sum % 10);
            tmpList = tmpList->next;
            
            head1 = head1->next;
        }
        
        while(head2 != NULL){
            int sum = carry + head2->val;
            carry = sum / 10;
            tmpList->next = new ListNode(sum % 10);
            tmpList = tmpList->next;
            
            head2 = head2->next;
        }
        
        if(carry == 1){
            tmpList->next = new ListNode(1);
        }
        tmpList = retList;
        retList = retList->next;
        
        delete tmpList;
        
        return retList;
    }
};

Anwser 2：