Add Two Numbers as Linked Lists

Add two non-negative integers stored as reverse-order digit linked lists and return the sum in the same format.

You are given two singly-linked lists, l1 and l2, each representing a non-negative integer. The digits are stored in reverse order: the head of each list holds the least-significant digit, and each node holds a single digit from 0 to 9.

Add the two numbers together and return the sum as a linked list, using the same reverse-digit format.

You may assume neither input represents a number with a leading zero, except for the number 0 itself (represented as a single node holding 0).

Example 1

Input: l1 = [2,4,3], l2 = [5,6,4]

Output: [7,0,8]

Explanation: 342 + 465 = 807, stored digit-by-digit in reverse order.

Example 2

Input: l1 = [9], l2 = [1]

Output: [0,1]

Explanation: 9 + 1 = 10, which needs an extra leading digit.

Example 3

Input: l1 = [0], l2 = [0]

Output: [0]

Constraints

The number of nodes in each list is in the range [1, 100].
0 <= Node.val <= 9
Neither list contains a leading zero, except the list [0] itself, which represents the number zero.

Follow-up

Could you solve this if the digits were stored in forward order (most-significant digit first) instead? How would your approach need to change?

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class Solution {
    public List<Integer> addTwoNumbers(ListNode l1, ListNode l2) {
        // your code here
        return new ArrayList<>();
    }
}

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