fn main() {
    let mut list = List::new();
    list.push(1);
    println!("push(1) : {:?}", list);
    list.push(2);
    println!("push(2) : {:?}", list);
    println!("pop() -> {} : {:?}", list.pop().unwrap(), list);
    println!("pop() -> {} : {:?}", list.pop().unwrap(), list);
}

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

type Link<T> = Option<Box<Node<T>>>;

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

impl<T> List<T> {
    pub fn new() -> Self {
        Self { head: None }
    }

    pub fn push(&mut self, elem: T) {
        let new_node = Node {
            elem: elem,
            next: self.head.take(),
        };
        self.head = Some(Box::new(new_node));
    }

    pub fn pop(&mut self) -> Option<T> {
        self.head.take().map(|node| {
            self.head = node.next;
            node.elem
        })
    }
}

#[cfg(test)]
mod tests {

    use super::*;

    #[test]
    fn test() {
        let mut list = List::new();
        assert_eq!(None, list.pop());
        list.push(42);
        assert_eq!(Some(42), list.pop());
        assert_eq!(None, list.pop());
    }


}