Method push

publication/code/chapter04/lvec.rs:51–68

(&mut self, elem: T)

    }

    fn push(&mut self, elem: T) {
        let node = Node::new(elem);
        if self.is_empty() {
            self.head = Some(Box::new(node));
        } else {
            let mut curr = self.head.as_mut().unwrap();

            // 找到链表最后一个节点
            for _i in 0..self.size-1 {
                curr = curr.next.as_mut().unwrap();
            }

            // 在最后一个节点后插入新数据
            curr.next = Some(Box::new(node));
        }

        self.size += 1;
    }

    // 栈末尾加入新的 LVec
    fn append(&mut self, other: &mut Self) {