二叉树的IntoIterator实现

Question

我正在尝试构建一个二叉树，并编写一个迭代器来遍历树中的值。当为我的树节点实现IntoIterator特性时，我遇到了一个生命周期问题

src\main.rs:43:6: 43:8 error: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates [E0207]
src\main.rs:43 impl<'a, T: 'a> IntoIterator for Node<T> {

我知道我需要说明NodeIterator的寿命和Node一样长，但我不知道该如何表达

use std::cmp::PartialOrd;
use std::boxed::Box;

struct Node<T: PartialOrd> {
    value: T,
    left: Option<Box<Node<T>>>,
    right: Option<Box<Node<T>>>,
}

struct NodeIterator<'a, T: 'a + PartialOrd> {
    current: &'a Node<T>,
    parent: Option<&'a Node<T>>,
}

impl<T: PartialOrd> Node<T> {
    pub fn insert(&mut self, value: T) {
        ...
    }
}

impl<'a, T: 'a> IntoIterator for Node<T> { // line 43
     type Item = T;
     type IntoIter = NodeIterator<'a, T>;

     fn into_iter(&self) -> Self::IntoIter {
         NodeIterator::<'a> {
             current: Some(&self),
             parent: None
         }
     }
 }

Answer 1

您所得到的特别错误是，'a应该出现在for的右侧。否则，编译器如何才能知道a是什么？

在实现IntoIterator时，您必须决定迭代器是否会消耗容器，或者它是否只生成对容器的引用。目前，您的设置不一致，错误消息指出了这一点。

在二叉树的情况下，您还必须考虑要生成值的顺序:传统顺序是深度优先(产生排序序列)和宽度优先(公开树的“层”)。我先假设深度，因为它是最常见的。

让我们首先处理消耗迭代器的情况。更简单的是，我们不需要担心生命的存在。

#![feature(box_patterns)]

struct Node<T: PartialOrd> {
    value: T,
    left: Option<Box<Node<T>>>,
    right: Option<Box<Node<T>>>,
}

struct NodeIterator<T: PartialOrd> {
    stack: Vec<Node<T>>,
    next: Option<T>,
}

impl<T: PartialOrd> IntoIterator for Node<T> {
    type Item = T;
    type IntoIter = NodeIterator<T>;

    fn into_iter(self) -> Self::IntoIter {
        let mut stack = Vec::new();

        let smallest = pop_smallest(self, &mut stack);

        NodeIterator { stack: stack, next: Some(smallest) }
    }
}

impl<T: PartialOrd> Iterator for NodeIterator<T> {
    type Item = T;

    fn next(&mut self) -> Option<T> {
        if let Some(next) = self.next.take() {
            return Some(next);
        }

        if let Some(Node { value, right, .. }) = self.stack.pop() {
            if let Some(right) = right {
                let box right = right;
                self.stack.push(right);
            }
            return Some(value);
        }

        None
    }
}

fn pop_smallest<T: PartialOrd>(node: Node<T>, stack: &mut Vec<Node<T>>) -> T {
    let Node { value, left, right } = node;

    if let Some(left) = left {
        stack.push(Node { value: value, left: None, right: right });
        let box left = left;
        return pop_smallest(left, stack);
    }

    if let Some(right) = right {
        let box right = right;
        stack.push(right);
    }

    value
}

fn main() {
    let root = Node {
        value: 3,
        left: Some(Box::new(Node { value: 2, left: None, right: None })),
        right: Some(Box::new(Node { value: 4, left: None, right: None }))
    };

    for t in root {
        println!("{}", t);
    }
}

现在，我们可以通过在适当的引用中添加适当的引用来“轻松”地将其适应于不消耗的情况：

struct RefNodeIterator<'a, T: PartialOrd + 'a> {
    stack: Vec<&'a Node<T>>,
    next: Option<&'a T>,
}

impl<'a, T: PartialOrd + 'a> IntoIterator for &'a Node<T> {
    type Item = &'a T;
    type IntoIter = RefNodeIterator<'a, T>;

    fn into_iter(self) -> Self::IntoIter {
        let mut stack = Vec::new();

        let smallest = pop_smallest_ref(self, &mut stack);

        RefNodeIterator { stack: stack, next: Some(smallest) }
    }
}

impl<'a, T: PartialOrd + 'a> Iterator for RefNodeIterator<'a, T> {
    type Item = &'a T;

    fn next(&mut self) -> Option<&'a T> {
        if let Some(next) = self.next.take() {
            return Some(next);
        }

        if let Some(node) = self.stack.pop() {
            if let Some(ref right) = node.right {
                self.stack.push(right);
            }
            return Some(&node.value);
        }

        None
    }
}

fn pop_smallest_ref<'a, T>(node: &'a Node<T>, stack: &mut Vec<&'a Node<T>>) -> &'a T
    where
        T: PartialOrd + 'a
{
    if let Some(ref left) = node.left {
        stack.push(node);
        return pop_smallest_ref(left, stack);
    }

    if let Some(ref right) = node.right {
        stack.push(right);
    }

    &node.value
}

里面有很多东西要打开，所以慢慢来消化吧。具体地说：

• ref在Some(ref right) = node.right中的使用是因为我不想使用node.right，而只是为了在Option中获得一个引用；编译器会抱怨没有它我无法从借来的对象中移出(所以我只是跟着抱怨)，
• 在stack.push(right)、right: &'a Box<Node<T>>和stack: Vec<&'a Node<T>>中；这是Deref的神奇之处：Box<T>实现了Deref<T>，因此编译器会根据需要自动转换引用。

注意:我没有按原样编写这段代码；相反，我只是将最初的几个引用放在我希望它们出现的地方(例如返回类型的Iterator__)，然后让编译器指导我。