add two numbers II

Java

public class Solution {
    /**
     * @param l1: the first list
     * @param l2: the second list
     * @return: the sum list of l1 and l2 
     */
    public ListNode addLists2(ListNode l1, ListNode l2) {
        l1 = reverse(l1);
        l2 = reverse(l2);
        
        return reverse(addList1(l1, l2));
    }  
    
    private ListNode reverse(ListNode head) {
        ListNode prev = null;
        while (head != null) {
            ListNode temp = head.next;
            head.next = prev;
            prev = head;
            head = temp;
        }
        return prev;
    }
    
    private ListNode addList1(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode tail = dummy;
        int carry = 0;
        
        while (l1 != null && l2 != null) {
            tail.next = new ListNode((l1.val + l2.val + carry) % 10);
            tail = tail.next;
            carry = (l1.val + l2.val + carry) / 10;
            
            l1 = l1.next;
            l2 = l2.next;
        }
        
        while (l1 != null) {
            tail.next = new ListNode((l1.val + carry) % 10);
            tail = tail.next;
            carry = (l1.val + carry) / 10;
            
            l1 = l1.next;
        }
        while (l2 != null) {
            tail.next = new ListNode((l2.val + carry) % 10);
            tail = tail.next;
            carry = (l2.val + carry) / 10;
            
            l2 = l2.next;
        }
        
        while (carry != 0) {
            tail.next = new ListNode(carry % 10);
            tail = tail.next;
            carry = carry / 10;
        }
        
        return dummy.next;
    }
}

Python

class Solution:
    def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
        num1,num2 = 0,0
        while l1 or l2:
            if l1:
                num1 = num1*10 + l1.val
                l1 = l1.next
            if l2:
                num2 = num2*10 + l2.val
                l2 = l2.next
        dummhead = ListNode(0)
        for num in str(num1+num2):
            newnode = ListNode(int(num))
            if not dummhead.next:
                dummhead.next = tail = newnode
            else:
                tail.next = newnode
                tail = tail.next
        
        return dummhead.next