pub struct Stack<T> {
    head: Link<T>,
}

enum Link<T> {
    Empty,
    More(Box<Node<T>>),
}

struct Node<T> {
    data: T,
    next: Link<T>,
}

impl<T> Stack<T> {
    pub fn new() -> Self {
        Stack { head: Link::Empty }
    }

    pub fn push(&mut self, data: T) {
        self.head = Link::More(Box::new(Node {
            data,
            next: std::mem::replace(&mut self.head, Link::Empty),
        }));
    }

    pub fn pop(&mut self) -> Option<T> {
        match std::mem::replace(&mut self.head, Link::Empty) {
            Link::Empty => None,
            Link::More(node) => {
                self.head = node.next;
                Some(node.data)
            }
        }
    }
}

impl<T> Iterator for Stack<T> {
    type Item = T;
    fn next(&mut self) -> Option<Self::Item> {
        self.pop()
    }
}