You are given two linked lists representing two non-negative numbers. The most significant digit comes first 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.

Follow up:
What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

Example:

Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 8 -> 0 -> 7

The key of this problem is to think of using Stack

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) { val = x; }
 7  * }
 8  */
 9 public class Solution {
10     public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
11         Stack<ListNode> st1 = new Stack<ListNode>();
12         Stack<ListNode> st2 = new Stack<ListNode>();
13         while (l1 != null) {
14             st1.push(l1);
15             l1 = l1.next;
16         }
17         while (l2 != null) {
18             st2.push(l2);
19             l2 = l2.next;
20         }
21         ListNode dummy = new ListNode(-1);
22         int carry = 0;
23         while (st1.size()!=0 || st2.size()!=0 || carry!=0) {
24             int sum = 0;
25             if (st1.size() != 0) sum += st1.pop().val;
26             if (st2.size() != 0) sum += st2.pop().val;
27             if (carry != 0) sum += carry;
28             ListNode newNode = new ListNode(sum%10);
29             newNode.next = dummy.next;
30             dummy.next = newNode;
31             carry = sum / 10;
32         }
33         return dummy.next;
34     }
35 }