2. Add Two Numbers

题目：

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

ListNode 在这里的构造函数是需要传入值的

操作的时候需要注意！！！：定义了head之后不要对head直接操作，要ListNode node = head；这样起别名，这样head永远是head，而剩下来的操作是对node来进行的。比如node = node.next;

/**
 * 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) {
        ListNode head = new ListNode(0);
        ListNode l3 = head;
        int carry = 0;
        while(l1!=null||l2!=null||carry!=0){
            if(l1==null) l1 = new ListNode(0);
            if(l2==null) l2 = new ListNode(0);
            int i = l1.val+l2.val+carry;
            if(i>=10){
                i=i-10;
                carry=1;
            }else{
                carry = 0;
            }
            l3.next = new ListNode(i);//因为l3是新创建的，所以每次加完就要创建一个新的next node并且赋值为i
            l3 = l3.next;
            l1 = l1.next;
            l2 = l2.next;
        }
        return head.next;
    }
}