use std::mem::{replace};

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

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

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

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

    pub fn push(&mut self, v: i32) {
        self.head = Link::Cons(Box::new(
            Node {
                element: v,
                next: replace(&mut self.head, Link::Nil)
            }
        ));
    }

    pub fn pop(&mut self) -> Option<i32> {
        match replace(&mut self.head, Link::Nil) {
            Link::Nil => None,
            Link::Cons(node) => {
                self.head = node.next;
                Some(node.element)
            }
        }
    }
}

fn main() {
    let mut l = List::new();
    l.push(19);
    l.push(20);

    println!("{:?}", l);
    println!("{:?}", l.pop());
    println!("{:?}", l);
}