Solution.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */

class Solution {
    
    // public class ListNode {
    //     int val;
    //     ListNode next;
    //     public ListNode(int x) { val = x; }
    // }
    
    // input is a ListNode with digits as elements
    
    public int pop(ListNode l) {
        
        int i = 1;
        int sum = 0;
        while(l.next != null) {
            sum += i*l.val;
            i *= 10;
        }
        
        return sum;
        
    }
    
    public ListNode num2list(int number) {
        ListNode l = null;
        
        while (number > 0) {
            l.val = number % 10;
            l = l.next;
            number = number / 10;
        }

        return l;
    }
    
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int n1 = pop(l1);
        int n2 = pop(l2);
    
        int n = n1 + n2;
        
        return num2list(n);
        
    }

    //public static void main (String[] args) {

    //}
}