[LeetCode]Add Two Numbers

题意：链表大数的加法处理，不过链表上的数是反序的。

本题来源：https://leetcode.com/problems/add-two-numbers/

分析：

1.如果其中链表为空，则不用计算了；如果两个链表都为空，则返回空（链表不熟）。

/**
 * 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(l1==NULL)return l2;
        if(l2==NULL)return l1;
        
        int carry = 0;
        ListNode* tail = new ListNode(0);
        ListNode* ptr = tail;
        
        while(l1 != NULL || l2 != NULL){
            int val1 = 0;
            if(l1 != NULL){
                val1 = l1->val;
                l1 = l1->next;
            }
            
            int val2 = 0;
            if(l2 != NULL){
                val2 = l2->val;
                l2 = l2->next;
            }
            
            int tmp = val1 + val2 + carry;
            ptr->next = new ListNode(tmp % 10);
            carry = tmp / 10;
            ptr = ptr->next;
        }
        if(carry == 1)ptr->next = new ListNode(1);
        return tail->next;
    }
};