/**

 * 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) {

        Stack<Integer> s1 = new Stack<Integer>();

        Stack<Integer> s2 = new Stack<Integer>();

        // store two numbers into stacks

        while(l1 != null) {

            s1.push(l1.val);

            l1 = l1.next;

        }

        while(l2 != null) {

            s2.push(l2.val);

            l2 = l2.next;

        }

        // adding two numbers from stacks

        int sum = 0;

        ListNode list = new ListNode(0);

        while(!s1.empty() || !s2.empty()) {

            if(!s1.empty())

                sum += s1.pop();

            if(!s2.empty())

                sum += s2.pop();

            list.val = sum % 10; // get the value

            ListNode head = new ListNode(sum / 10); // get the carry

            head.next = list;

            list = head;

            sum /= 10;

        }

        // need to check if the head is 0 or not

        return list.val == 0 ? list.next : list;

    }

}