Question

What is the complexity of this algorithm? I am assuming it’s O(N) but I’d like some clarification.
If I had a tail in the linked list, I would assume this would be even faster since I would completely avoid having that while(current != null) loop to loop to the end of the list.

public void reverse() {
    Stack<Node> stack = new Stack<Node>();
    if (this.head != null || this.head.next != null) {
        Node current = this.head;
        while (current != null) {
            stack.push(current);
            current = current.next;
        }
        Node last = null;
        while (!stack.empty()) {
            if (last==null) {
                this.head = stack.pop();
                last = this.head;
                continue;
            }
            last.next = stack.pop();
            last = last.next;
            if (stack.empty()) {
                last.next = null;
            }
        }
    }
}

Answer 1

You are pushing all the linked list elements onto stack by iterating the list this is one N then you iterate the stack which is another N so you Order notation in O(2N)

see how to reverse a list with O(1) space and O(n) time?