use std::mem;


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

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

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


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

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

		    self.head = Link::More(new_node);
		}

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


    }
}