Grokking 223-2

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int len1 = 0, len2 = 0, carry = 0;

        // Get length of l1 and l2
        ListNode curr = l1;
        while (curr != null) {
            len1++;
            curr = curr.next;
        }

        curr = l2;
        while (curr != null) {
            len2 ++;
            curr = curr.next;
        }

        // Ensure length of l1 > length of l2
        if (len1 < len2) {
            curr = l1;
            l1 = l2;
            l2 = curr;

            int tmp = len1;
            len1 = len2;
            len2 = tmp;
        }

        carry = helper(l1, l2, len1, len2);
        if (carry > 0) {
            return new ListNode(carry, l1);
        }

        return l1;
    }

    int helper(ListNode l1, ListNode l2, int len1, int len2) {
        if (l1 == null) {
            return 0;
        }

        int n1 = l1.val;
        int n2 = l2.val;
        int carry = 0;

        if (len1 > len2) {
            n2 = 0;
            carry = helper(l1.next, l2, len1 - 1, len2);
        } else {
            carry = helper(l1.next, l2.next, len1 - 1, len2);
        }

        l1.val = n1 + n2 + carry;
        carry = l1.val / 10;
        l1.val %= 10;

        return carry;
    }
}