rust - while 循环中变量的突变

Question

我正在尝试构建一个单链表:

pub struct Node<T> {
    pub element: T,
    pub next: Option<Box<Node<T>>>,
}

struct List<T> {
    head: Option<Node<T>>,
}

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

    fn add(&mut self, element: T) {
        let node = Node {
            next: None,
            element,
        };
        // get the node at the end of the list
        match self.head {
            None => self.head = Some(node),
            Some(_) => {
                let mut last_node: Option<Box<&Node<T>>> =
                    Some(Box::new(self.head.as_ref().unwrap()));
                while let Some(node) = last_node {
                    let n = node.next.take().map(|n| &n);
                    last_node = n;
                }
            }
        }
    }
}

fn main() {}

我遇到编译错误:

error[E0308]: mismatched types
  --> src/main.rs:28:33
   |
28 |                     last_node = n;
   |                                 ^ expected struct `std::boxed::Box`, found reference
   |
   = note: expected type `std::option::Option<std::boxed::Box<&Node<T>>>`
              found type `std::option::Option<&std::boxed::Box<Node<T>>>`

虽然我理解错误背后的原因，但我无法解决这个问题。

我需要在列表的最后添加一个元素。为此，我采用了重复循环列表的下一个元素的方法，直到它不是 None 并在那里设置新值。

我知道这是我们在 C 或 Java 中执行的命令式方法。我不确定如何在 Rust 中执行此操作。

Answer 1

我将按如下方式实现 add 方法。

fn add(&mut self, element: T) {
    let node = Node { element, next: None };
    match self.head {
        None => {
            self.head = Some(node);
        }
        Some(ref mut head) => {
            let mut cur = head;
            while cur.next.is_some() {
                cur = {cur}.next.as_mut().unwrap();
            }
            cur.next = Some(Box::new(node));
        }
    }
}

测试实现:

fn main() {
    let mut list = List::new();
    println!("{:?}", list);

    list.add(5);
    println!("{:?}", list);

    list.add(4);
    println!("{:?}", list);

    list.add(3);
    println!("{:?}", list);
}

输出是:

List { head: None }
List { head: Some(Node { element: 5, next: None }) }
List { head: Some(Node { element: 5, next: Some(Node { element: 4, next: None }) }) }
List { head: Some(Node { element: 5, next: Some(Node { element: 4, next: Some(Node { element: 3, next: None }) }) }) }