Why do I need if node after while queue to invert binary tree

Question

I was trying to invert a binary tree (i.e. swap all left & right children)

class Node(object):
    def __init__(self, value=None, left=None, right=None):
        self.value = value
        self.left = left
        self.right = right
    
    def __repr__(self):
        return f"Node({self.value}, {self.left}, {self.right})"

from collections import deque

def invert(root):
    q=deque([root])
    while q:
        n=q.popleft() #n as in node
        if n: #why do I need this
            n.left,n.right=n.right,n.left
            q.append(n.left)
            q.append(n.right)
    return root

root = Node(1, Node(2), Node(3))
invert(root)

I realized if I remove if n, I get NoneType error. I was thinking if while q is true, then if n will always be true so if n is redundant code.

Asked By: user35897

||

Source

Answer 1

It is needed.

First of all it is needed when the tree is empty, because then that q=deque([root]) will have put a None value on the queue, which should be ignored in the loop.

But also when the tree is not empty it is needed:

When a node n does not have both left and right children, then one (or both) of the q.append calls will append a None value on the queue. And so in one of the next iterations of the loop this None value will be popped from the queue. But in that case nothing should happen.

So yes, the if is needed.

You can rearrange the code, but somewhere you’ll have to detect None values. For instance, take this variant:

(NB: the comment "n as in node" indicates you should just name it node instead of n)

if not root:
    return
queue = deque([root])
while queue:
    node = queue.popleft()
    node.left, node.right = node.right, node.left
    if node.left:
        queue.append(node.left)
    if node.right:
        queue.append(node.right)
return root

This version avoids that the queue would ever receive a None value, but it still means you need an if — in this case three of them!

Answered By: trincot

Why do I need if node after while queue to invert binary tree

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