LeetCode - Add Two Numbers

题目描述：

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

方法一：

解题思路：建立一个空链表tail，用一个变量表示进位，然后用两个指针变量遍历l1和l2，分别求和赋值给tail.

/** * 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 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;    }  };