Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order 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.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

time o(n), space o(n)

/**
 * 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) {
        
        
        int sum = 0;
        
        ListNode dummy = new ListNode(0);
        ListNode cur = dummy;
        
        while(l1 != null || l2 != null){
            if(l1 != null){
                sum += l1.val;
                l1 = l1.next;
            }
            
            if(l2 != null){
                sum += l2.val;
                l2 = l2.next;
            }
            
            cur.next = new ListNode(sum % 10);
            sum/=10;
            
            cur = cur.next;
        }
        
        if(sum != 0 ){
            cur.next = new ListNode(1);
        }
        
        return dummy.next;

        
       
    }
}