[LintCode] Add Two Numbers 两个数字相加

You have two numbers represented by a linked list, where each node contains a single digit. The digits are stored in reverse order, such that the 1's digit is at the head of the list. Write a function that adds the two numbers and returns the sum as a linked list.

Have you met this question in a real interview? 

Yes

Example

Given 7->1->6 + 5->9->2. That is, 617 + 295.

Return 2->1->9. That is 912.

Given 3->1->5 and 5->9->2, return 8->0->8.

LeetCode上的原题，请参见我之前的博客Add Two Numbers。

class Solution {
public:
    /**
     * @param l1: the first list
     * @param l2: the second list
     * @return: the sum list of l1 and l2 
     */
    ListNode *addLists(ListNode *l1, ListNode *l2) {
        ListNode *dummy = new ListNode(-1), *cur = dummy;
        int carry = 0;
        while (l1 || l2) {
            int num1 = l1 ? l1->val : 0;
            int num2 = l2 ? l2->val : 0;
            int sum = num1 + num2 + carry;
            cur->next = new ListNode(sum % 10);
            carry = sum / 10;
            if (l1) l1 = l1->next;
            if (l2) l2 = l2->next;
            cur = cur->next;
        }
        if (carry) cur->next = new ListNode(1);
        return dummy->next;
    }
};