2. Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. 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.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

此题和add binary比较类似，只不过不用再考虑数组的高位是num的低位了，代码如下：

/**

 * Definition for singly-linked list.

 * public class ListNode {

 *     int val;

 *     ListNode next;

 *     ListNode(int x) { val = x; }

 * }

 */

public class Solution {

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {

        int carry = 0;

        ListNode res = new ListNode(0);

        ListNode dummy = res;

        while(l1!=null||l2!=null||carry!=0){

            if(l1!=null){

                carry+=l1.val;

                l1 = l1.next;

            }

            if(l2!=null){

                carry+=l2.val;

                l2 = l2.next;

            }

            res.val = carry%10;

            carry/=10;

            if(l1!=null||l2!=null||carry!=0){

                ListNode next = new ListNode(0);

                res.next = next;

                res = res.next;

            }

        }

        return dummy;

    }

}