You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first 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.

Follow up:
What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

Example:

Input:  (7 - > 2 - > 4 - > 3) + (5 - > 6 - > 4)

Output: 7 - > 8 - > 0 - > 7

Solution 1: 用2个stack压入2个list，从而可以从尾部开始加

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Deque<Integer> stack1 = new LinkedList<>();
        Deque<Integer> stack2 = new LinkedList<>();
        while (l1 != null) {
            stack1.offerLast(l1.val);
            l1 = l1.next;
        }
        while (l2 != null) {
            stack2.offerLast(l2.val);
            l2 = l2.next;
        }
        int extra = 0;
        ListNode prev = null;
        while (!stack1.isEmpty() && !stack2.isEmpty()) {
            int sum = stack1.pollLast() + stack2.pollLast() + extra;
            extra = sum / 10;
            sum %= 10;
            ListNode cur = new ListNode(sum);
            cur.next = prev;
            prev = cur;
        }
        while (!stack1.isEmpty() || !stack2.isEmpty()) {
            int sum = !stack1.isEmpty() ? stack1.pollLast() + extra : stack2.pollLast() + extra;
            extra = sum / 10;
            sum %= 10;
            ListNode cur = new ListNode(sum);
            cur.next = prev;
            prev = cur;
        }
        if (extra > 0) {
            ListNode cur = new ListNode(extra);
            cur.next = prev;
            prev = cur;
        }
        return prev;
    }

Solution 2: recursion

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int size1 = getLength(l1);
        int size2 = getLength(l2);
        ListNode head = new ListNode(1);
        // Make sure l1.length >= l2.length
        head.next = size1 < size2 ? helper(l2, l1, size2 - size1) : helper(l1, l2, size1 - size2);
        // Handle the first digit
        if (head.next.val > 9) {
            head.next.val = head.next.val % 10;
            return head;
        }
        return head.next;
    }
    // get length of the list
    public int getLength(ListNode l) {
        int count = 0;
        while(l != null) {
            l = l.next;
            count++;
        }
        return count;
    }
    // offset is the difference of length between l1 and l2
    public ListNode helper(ListNode l1, ListNode l2, int offset) {
        if (l1 == null) return null;
        // check whether l1 becomes the same length as l2
        ListNode result = offset == 0 ? new ListNode(l1.val + l2.val) : new ListNode(l1.val);
        ListNode post = offset == 0 ? helper(l1.next, l2.next, 0) : helper(l1.next, l2, offset - 1);
        // handle carry 
        if (post != null && post.val > 9) {
            result.val += 1;
            post.val = post.val % 10;
        }
        // combine nodes
        result.next = post;
        return result;
    }