从递归算法构建生成器

Question

我有一个递归方法，我用来走过一棵红黑树，并存储各种节点信息（在列表storage中）。

def _walk (self, storage, func, starting_node) :
    if starting_node is not self._nil :
        self._walk(storage, func, starting_node.left)
        storage.append(func(starting_node))
        self._walk(storage, func, starting_node.right)

但是，我想重新实现这个方法，以便它构建一个生成器（根据我的理解，这应该节省时间和内存）。这样做的“最佳”方式是什么？

Answer 1

首先将动作与行走分开

def _walk (self, starting_node) :
    if starting_node is not self._nil :
        for x in self._walk(starting_node.left):
            yield x
        yield starting_node
        for x in self._walk(starting_node.right):
            yield x

def traverse(self):
    starting_node = ???     # maybe these are passed as
    func = ???              # parameters to traverse
    for node in self._walk(starting_node):
        yield func(node)

traverse大致相当于

imap(func, self._walk(starting_node))

或此生成器表达式

(func(x) for x in self._walk(starting_node))

---

您可以通过手动优化尾递归来减少堆栈使用量

def _walk (self, starting_node) :
    while starting_node is not self._nil :
        for x in self._walk(starting_node.left):
            yield x
        yield starting_node
        starting_node = starting_node.right

Answer 2

def _walk (self, func, starting_node) :
    if starting_node is not self._nil :
        for x in self._walk(func, starting_node.left) :
            yield x
        yield func(starting_node)
        for x in self._walk(func, starting_node.right) :
            yield x