Question

我正在尝试在python中创建一个模块，使用4个标准树遍历（有序，前序，后序和水平顺序）迭代二叉树，而不使用条件，只使用多态方法dispatch或迭代器。以下示例应该有效。

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，我已经提出了以下内容

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其中一个迭代器时，我得到了一个 AttributeError：'NoneType'对象没有'inorder'属性 错误信息。我想这是因为我从不提出StopIteration。关于如何解决这个问题并开始实施levelorder的任何想法？

Answer 1

你说你想使用多态，但实际上你似乎并没有这样做。使用支持方法的特殊对象替换代码中所有出现的“无”，但返回一个空序列，它将全部起作用。

在发布Python问题时，您还应该更加注意缩进。您发布的代码不会按原样运行。

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)

Python：二叉树遍历迭代器而不使用条件

1 个答案: