Python: binary tree traversal iterators without using conditionals

Question

I am trying to create a module in python for iterating over a binary tree using the 4 standard tree traversals (inorder, preorder, postorder and levelorder) without using conditionals and only using polymorphic method dispatch or iterators. The following examples should work.

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)

So far I have come up with the following

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)

When I try to run one of the iterators like at the bottom of the code I get an AttributeError: 'NoneType' object has no attribute 'inorder' error message. I think this is because I never raise StopIteration. Any ideas on how to fix this and start implementing levelorder?

Answer 1

You said you wanted to use polymorphism, but you don't actually seem to have done so. Replace all occurrences of 'None' in your code with a special object that supports your methods but returns an empty sequence and it will all work.

Also you should take more care of the indentation when posting Python questions. The code you posted won't run as is.

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)