題目：

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

分析：

1. 需要記錄上一位的進位。

2. 考慮兩個連結串列的長度不同。

Java程式碼實現：

/**
 * 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) {
        if(l1==null)
            return l2;
        if(l2==null)
            return l1;
            
        int last = 0;
        ListNode dummy = new ListNode(0);
        ListNode node = dummy;
        
        while(l1!=null && l2!=null)
        {
            node.next = new ListNode((l1.val+l2.val+last)%10);
            last = (l1.val+l2.val+last)/10;
            l1 = l1.next;
            l2 = l2.next;
            node = node.next;
        }
        
        while(l1!=null)
        {
            node.next = new ListNode((l1.val+last)%10);
            last = (l1.val+last)/10;
            l1 = l1.next;
            node = node.next;
        }
        
        while(l2!=null)
        {
            node.next = new ListNode((l2.val+last)%10);
            last = (l2.val+last)/10;
            l2 = l2.next;
            node = node.next;
        }
        
        if(last!=0)
        {
            node.next = new ListNode(last);
            node = node.next;
        }
        node.next = null;
        
        return dummy.next;
    }
}