Python：无条件的二叉树遍历迭代器

Question

我正在尝试用Python创建一个模块，用来遍历二叉树，使用四种标准的遍历方式（中序遍历、前序遍历、后序遍历和层序遍历），而且不想用条件判断，只想用多态方法调用或者迭代器。下面的例子应该可以正常工作。

for e in t.preorder():
  print(e)
for e in t.postorder():
  print(e)
for e in t.inorder():
  print(e)
for e in t.levelorder():
  print(e)

到目前为止，我想出了以下内容

def build_tree(preord, inord):
  tree = BinaryTree()
  tree.root = buildTreeHelper(preord, inord)
  return tree

def buildTreeHelper(preorder, inorder):
  if len(inorder) == 0:
    return None

  elem = preorder[0]
  elemInorderIndex = inorder.find(elem)

  if elemInorderIndex > -1:
    leftPreorder = preorder[1:elemInorderIndex + 1]
    rightPreorder = preorder[elemInorderIndex + 1:]
    leftInorder = inorder[0:elemInorderIndex]
    rightInorder = inorder[elemInorderIndex + 1:]
    left = buildTreeHelper(leftPreorder, leftInorder)
    right = buildTreeHelper(rightPreorder, rightInorder)
    return BinaryTreeNode(elem, left, right)
  else:
    return "No valid tree for the given args"

class BinaryTree:
    def __init__(self):
        self.root = None
    def preorder(self):
        return self.root.preorder()
    def inorder(self):
        return self.root.inorder()
    def postoder(self):
        return self.root.postorder()

class BinaryTreeNode:
    def __init__(self, element, left=None, right=None):
        self.element = element
        self.left = left
        self.right = right
    def preorder(self):
        yield self.element
        for e in self.left.preorder():
            yield e
        for e in self.right.preorder():
            yield e
    def inorder(self):
        for e in self.left.inorder():
            yield e
        yield self.element
        for e in self.right.inorder():
            yield e
    def postorder(self):
        for e in self.left.postorder():
            yield e
        for e in self.right.postorder():
            yield e
        yield self.element

if __name__ == "__main__":
    t = build_tree("BAC", "ABC")
    for e in t.inorder():
        print(e)

当我尝试运行代码底部的某个迭代器时，出现了一个 AttributeError: 'NoneType' object has no attribute 'inorder' 错误信息。我觉得这是因为我从来没有抛出StopIteration异常。有没有什么想法可以解决这个问题，并开始实现层序遍历？

Answer 1

你说你想用多态，但看起来你并没有真正做到这一点。把你代码里所有的'None'替换成一个特殊的对象，这个对象可以支持你的方法，但返回一个空的序列，这样就可以正常工作了。

另外，发Python问题的时候要注意缩进。你发的代码现在是不能运行的。

def build_tree(preord, inord):
    tree = BinaryTree()
    tree.root = buildTreeHelper(preord, inord)
    return tree

def buildTreeHelper(preorder, inorder):
    if len(inorder) == 0:
        return empty

    elem = preorder[0]
    elemInorderIndex = inorder.find(elem)

    if elemInorderIndex > -1:
        leftPreorder = preorder[1:elemInorderIndex + 1]
        rightPreorder = preorder[elemInorderIndex + 1:]
        leftInorder = inorder[0:elemInorderIndex]
        rightInorder = inorder[elemInorderIndex + 1:]
        left = buildTreeHelper(leftPreorder, leftInorder)
        right = buildTreeHelper(rightPreorder, rightInorder)
        return BinaryTreeNode(elem, left, right)
    else:
        return "No valid tree for the given args"

class BinaryTree:
    def __init__(self):
        self.root = empty
    def preorder(self):
        return self.root.preorder()
    def inorder(self):
        return self.root.inorder()
    def postorder(self):
        return self.root.postorder()

class EmptyNode:
    def preorder(self):
        return ()
    inorder = postorder = preorder
empty = EmptyNode()

class BinaryTreeNode:
    def __init__(self, element, left=empty, right=empty):
        self.element = element
        self.left = left
        self.right = right
    def preorder(self):
        yield self.element
        for e in self.left.preorder():
            yield e
        for e in self.right.preorder():
            yield e
    def inorder(self):
        for e in self.left.inorder():
            yield e
        yield self.element
        for e in self.right.inorder():
            yield e
    def postorder(self):
        for e in self.left.postorder():
            yield e
        for e in self.right.postorder():
            yield e
        yield self.element

if __name__ == "__main__":
    t = build_tree("BAC", "ABC")
    for e in t.inorder():
        print(e)