#2.两数相加AddTwoNumbers  

给定两个非空链表来代表两个非负数，位数按照逆序方式存储，它们的每个节点只存储单个数字。将这两数相加会返回一个新的链表。你可以假设除了数字 0 之外，这两个数字都不会以零开头。

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.

示例：

输入：(2 -> 4 -> 3) + (5 -> 6 -> 4)
输出：7 -> 0 -> 8
原因：342 + 465 = 807

个人思路：

声明新链表及其指针和进位，设定循环，每轮的总求和为两链表指针当前位置的数字之和再加上一轮的进位，计算当前结果值为总求和的个位，进位值为其十位，直到两个链表均为空

代码（Java）：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        boolean end_flag_1=false,end_flag_2=false;
        ListNode list1 = l1;
        ListNode list2 = l2;
        ListNode result = new ListNode(0);
        ListNode head = result;
        int add = 0;
        while(true){
            int sum = list1.val + list2.val + add;
            head.val = sum%10;
            add = sum/10;         
            if(list1.next==null&&list2.next==null){
                if(add!=0){
                    head.next = new ListNode(add);
                }
                break;
            }
            if(list1.next==null){
                list1.next = new ListNode(0);
            }
            else if(list2.next==null){
                list2.next = new ListNode(0);
            }
            list1 = list1.next;
            list2 = list2.next;
            head.next = new ListNode(0);
            head = head.next;  
        }
        return result;
    }
}