https://leetcode.com/problems/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.

Example:
```
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.
```

 对于这个问题还有一个很好的方法：

1、将两个链表逆序，这样就可以依次得到从低到高位的数字

2、同步遍历两个逆序后链表，相加生成新链表，同时关注进位

3、当两个链表都遍历完成后，关注进位。

4、将两个逆序的链表再逆序一遍，调整回去


```java
public class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
}
 
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode  listNode= new ListNode(0);
        ListNode p = listNode;
        int sum = 0;
 
        while (l1 != null || l2 != null || sum != 0) {
            if (l1 != null) {
                sum += l1.val;
                l1 = l1.next;
            }
            if (l2 != null) {
                sum += l2.val;
                l2 = l2.next;
            }
            p.next = new ListNode(sum % 10);
            sum = sum / 10;
            p = p.next;
        }
        return listNode.next;
    }
}
```

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