Python：二叉树遍历迭代器不使用条件

Question

我想在python中创建一个模块，使用4标准树遍历（inorder，preorder，postorder和levelorder）遍历二叉树而不使用条件并只使用多态方法分派或迭代器。下面的例子应该工作。

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

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) 

当我尝试运行迭代的一个类似的代码的底部，我得到一个 AttributeError的：“NoneType”对象没有属性'inorder' 错误消息。我认为这是因为我从来没有提出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，但实际上你并没有这样做。将代码中所有出现的“无”替换为支持您的方法的特殊对象，但返回一个空序列，它将全部工作。

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)