## ้ข็ฎๆ่ฟฐ

``````You are given two non-empty linked lists representing two non-negative integers. 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.

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``````

## ไปฃ็ 

### Approach #1 Two Stack

``````/**
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
Stack<Integer> s1 = new Stack<Integer>();
Stack<Integer> s2 = new Stack<Integer>();

while (l1 != null) {
s1.push(l1.val);
l1 = l1.next;
}

while (l2 != null) {
s2.push(l2.val);
l2 = l2.next;
}

int sum = 0;
while (!s1.empty() || !s2.empty()) {
if (!s1.empty())    sum += s1.pop();
if (!s2.empty())    sum += s2.pop();