fn main() {}

#[derive(Debug)]
pub struct List {
    head: Link,
}

#[derive(Debug)]
enum Link {
    Empty,
    More(Box<Node>),
}

#[derive(Debug)]
struct Node {
    elem: i32,
    next: Link,
}

impl List {
    pub fn new() -> Self {
        Self { head: Link::Empty }
    }

    pub fn push(&mut self, elem: i32) {
        let new_node = Node {
            elem: elem,
            next: std::mem::replace(&mut self.head, Link::Empty),
        };
        self.head = Link::More(Box::new(new_node));
    }
}