LeetCode Add Two Numbers

题目：

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

/**
 * 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 flag = 0, tmp = 0;
        ListNode *sum = l1;
        ListNode *pre = l1;
        while(l1 != NULL && l2 != NULL) {
            tmp = flag + l1->val + l2->val;
            if(tmp >= 10) {
                flag = 1;
                tmp -= 10;
            }
            else
                flag = 0;
            l1->val = tmp;
            pre = l1;
            l1 = l1->next;
            l2 = l2->next;
        }
        //链表1有剩余
        while(l1 != NULL && flag == 1) {
            tmp = flag + l1->val;
            if(tmp >= 10) {
                flag = 1;
                tmp -= 10;
            }
            else
                flag = 0;
            l1->val = tmp;
            pre = l1;
            l1 = l1->next;
        }
        if(l2 != NULL)
            pre->next = l2;
        //链表2有剩余
        while(l2 != NULL && flag == 1) {
            tmp = flag + l2->val;
            if(tmp >= 10) {
               flag = 1;
               tmp -= 10;
            }
            else
                flag = 0;
            l2->val = tmp;
            pre = l2;
            l2 = l2->next;
        }
        //处理最后的进位
        if(flag)
        	pre->next = new ListNode(1);
        return sum;
    }
};