DFS

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int len1 = getLength(l1);
        int len2 = getLength(l2);

        int offset = Math.abs(len1 - len2);
        ListNode head = len1 > len2 ? helper(l1, l2, offset) : helper(l2, l1, offset);

        if (head != null && head.val >= 10) {
            ListNode headhead = new ListNode(head.val / 10);
            headhead.next = head;
            head.val = head.val % 10;

            return headhead;
        }

        return head;
    }

    private ListNode helper(ListNode l1, ListNode l2, int offset) {
        if (l1 == null && l2 == null) return null;

        ListNode node = offset > 0 ? l1 : new ListNode(l1.val + l2.val);
        ListNode next = offset > 0 ? helper(l1.next, l2, offset - 1) : helper(l1.next, l2.next, 0);

        int carry = 0;
        if (next != null && next.val >= 10) {
            carry = next.val / 10;
            next.val = next.val % 10;
        }

        node.val = node.val + carry;
        node.next = next;

        return node;
    }

    private int getLength(ListNode node) {
        int len = 0;

        while (node != null) {
            len++;
            node = node.next;
        }

        return len;
    }
}

Stack

public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Stack<ListNode> s1 = new Stack<>();
        Stack<ListNode> s2 = new Stack<>();

        while (l1 != null) {
            s1.push(l1);
            l1 = l1.next;
        }

        while (l2 != null) {
            s2.push(l2);
            l2 = l2.next;
        }
        int carry = 0;
        ListNode dummyNode = new ListNode(0);
        while (!s1.isEmpty() || !s2.isEmpty()) {
            ListNode node = new ListNode((s1.isEmpty() ? 0 : s1.pop().val) + (s2.isEmpty() ? 0 : s2.pop().val) + carry);

            carry = node.val / 10;
            node.val = node.val % 10;

            node.next = dummyNode.next;
            dummyNode.next = node;
        }

        if (carry != 0) {
            ListNode node = new ListNode(carry);
            node.next = dummyNode.next;
            dummyNode.next = node;
        }

        return dummyNode.next;
    }
}