/**

 * 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) {

        ListNode dummyHead = new ListNode(0);

        ListNode cursor1 = l1;

        ListNode cursor2 = l2;

        ListNode cursor3 = dummyHead;

        int carry = 0;

        while(cursor1 != null || cursor2 != null) // go through both list

        {

            // if node exists, get the value, elsewise, default 0

            int num1 = 0;

            int num2 = 0;

            if(cursor1 != null)

                num1 = cursor1.val;

            if(cursor2 != null)

                num2 = cursor2.val;

            int sum = num1 + num2 + carry;

            // update carry and sum

            carry = sum / 10;

            cursor3.next = new ListNode(sum % 10);

            cursor3 = cursor3.next;

            // move both list to next node

            if(cursor1 != null)

                cursor1 = cursor1.next;

            if(cursor2 != null)

                cursor2 = cursor2.next;

        }

        // at last, still need to check carry for last digit

        if(carry == 1)

            cursor3.next = new ListNode(1);

        return dummyHead.next;

    }

}