Method insert

code/chapter08/dfs.rs:53–62

(&mut self, data: usize)

    }

    fn insert(&mut self, data: usize) {
        let node = Rc::new(RefCell::new(Node::new(data)));
        if self.is_empty() {
            self.first = Some(node.clone());
            self.last = Some(node);
        } else {
            self.last.as_mut().unwrap().borrow_mut().next = Some(node.clone());
            self.last = Some(node);
        }
    }
}

// 构建图