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

题目的意思是给定两个单向链表，链表中每个节点都存这一位数，将两个链表相加，返回一个新的链表。

解决这道题我们主要是要处理进位，用一个变量carry来维护进位，遍历两个链表，因为要返回一个新链表，因此我们要保存头结点，我们用一个辅助节点helper来保存头结点。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 helper = new ListNode(0);
        ListNode head = helper;
        while(l1 != null || l2 != null) {
            if(l1 != null) {
                carry += l1.val;
                l1 = l1.next;
            }
            if(l2 != null) {
                carry += l2.val;
                l2 = l2.next;
            }
            head.next = new ListNode(carry % 10);
            head = head.next;
            carry /= 10;
        }
        if(carry == 1) {
            head.next = new ListNode(carry);
        }
        return helper.next;
    }
}