leetcode: Add Two Numbers

http://oj.leetcode.com/problems/add-two-numbers/

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

思路：

注意以下两点，其它都简单：

1. 两个链表长度不一样。
2. 进位的处理。

class Solution {
public:
    void insertNode(ListNode *node, ListNode *&head, ListNode *&tail) {
        if (NULL == head) {
            head = tail = node;
        }
        else {
            tail->next = node;
            tail = node;
        }
    }

    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        ListNode *head = NULL, *tail = NULL;
        int sum, flag = 0;

        while ((l1 != NULL) || (l2 != NULL)) {
            sum = ((l1 != NULL) ? l1->val : 0) + ((l2 != NULL) ? l2->val : 0) + flag;
            flag = sum / 10;
            insertNode(new ListNode(sum % 10), head, tail);

            l1 = (l1 != NULL) ? l1->next : NULL;
            l2 = (l2 != NULL) ? l2->next : NULL;
        }

        if (flag > 0) {
            insertNode(new ListNode(flag), head, tail);
        }

        return head;
    }
};