/**
 * Definition for singly-linked list.
 * public class ListNode {
 *  public int val;
 *  public ListNode next;
 *  public ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode AddTwoNumbers(ListNode l1, ListNode l2) {
        if(l1 == null || l2 == null) return l1;
        ListNode head = new ListNode(0);//return head.next later
        ListNode current = head;
        int flag = 0;
        while(l1 != null || l2 != null || flag == 1){
            int temp = (l1 != null ? l1.val : 0) + (l2 != null ? l2.val : 0) + flag;
            current.next = new ListNode(temp % 10);
            flag = temp / 10;
            current = current.next;
            l1 = l1 != null ? l1.next : l1;
            l2 = l2 != null ? l2.next : l2;
        }
        return head.next;
    }
}