遍历 BST 时借用时临时值下降

Question

use std::{cell::RefCell, rc::Rc};

#[derive(Debug, PartialEq, Eq)]
pub struct TreeNode {
    pub val: i32,
    pub left: Option<Rc<RefCell<TreeNode>>>,
    pub right: Option<Rc<RefCell<TreeNode>>>,
}

fn find_min(mut node: &Option<Rc<RefCell<TreeNode>>>) -> i32 {
    while node.as_ref().unwrap().borrow().left.is_some() {
        node = &node.as_ref().unwrap().borrow().left;
    }
    node.as_ref().unwrap().borrow().val
}

error[E0716]: temporary value dropped while borrowed
  --> src/lib.rs:12:17
   |
10 | fn find_min(mut node: &Option<Rc<RefCell<TreeNode>>>) -> i32 {
   |                       - let's call the lifetime of this reference `'1`
11 |     while node.as_ref().unwrap().borrow().left.is_some() {
12 |         node = &node.as_ref().unwrap().borrow().left;
   |         --------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^------ temporary value is freed at the end of this statement
   |         |       |
   |         |       creates a temporary which is freed while still in use
   |         assignment requires that borrow lasts for `'1`

TreeNode 已给定且无法更改。

我明白为什么会出现错误，但不知道如何解决。

Answer 1

更新：

使用 Rc 作为参数看起来可能更好：

use std::rc::Rc;

#[derive(Debug, PartialEq, Eq)]
pub struct TreeNode {
    pub val: i32,
    pub left: Option<Rc<TreeNode>>,
    pub right: Option<Rc<TreeNode>>,
}

pub fn find_min(mut current: Rc<TreeNode>) -> i32 {
    while current.left.is_some() {
        current = Rc::clone(current.left.as_ref().unwrap());
    }
    current.val
}

fn main() {
    let left_child = TreeNode {
        val: 1,
        left: None,
        right: None,
    };
    let root = TreeNode {
        val: 6,
        left: Some(Rc::new(left_child)),
        right: None,
    };
    println!("min: {}", find_min(Rc::new(root)));
}

RefCell 和 mut 对我来说是多余的？

怎么样：

use std::rc::Rc;

#[derive(Debug, PartialEq, Eq)]
pub struct TreeNode {
    pub val: i32,
    pub left: Option<Rc<TreeNode>>,
    pub right: Option<Rc<TreeNode>>,
}

pub fn find_min(root: &TreeNode) -> i32 {
    let mut current = Rc::new(root);
    while current.left.is_some() {
        current = Rc::new(current.left.as_ref().unwrap());
    }
    current.val
}

Answer 2

根据写了我的解决方案：

fn find_min(node: &Option<Rc<RefCell<TreeNode>>>) -> i32 {
    let mut curr = node.as_ref().unwrap().clone();
    while curr.borrow().left.is_some() {
        let tmp = curr.borrow().left.as_ref().unwrap().clone();
        curr = tmp;
    }
    let res = curr.borrow().val;
    res
}

两个临时变量看起来都不太好，但就是这样。

遍历 BST 时借用时临时值下降

Temporary value dropped while borrowed when traversing a BST

rust

binary-search-tree