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I am writing code to solve the following leetcode problem: https://leetcode.com/problems/symmetric-tree/

The problem in a nutshell is "Given the root of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center)."

from collections import deque
class Solution:
    def isSymmetric(self, root: TreeNode) -> bool:
        queue= deque()
        if not root:
            return []
        #add left and right child of root to start (if root is not None)
        if root.left: 
            queue.append(root.left)
        if root.right: 
            queue.append(root.right)

        right_subt = []
        left_subt = []

        while queue:
            level_length = len(queue)
            #iterate over each level of the tree and add left subtree to left_subt and right subtree to right_subt

            for _ in range((level_length//2)+1):
                node = queue.popleft()
                left_subt.append(node.val)
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
            for _ in range((level_length-(level_length//2))+1):
                node = queue.popleft()
                right_subt.append(node.val)
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
            #compare left and right subtree at each level to check if they're the same
            if left_subt !=right_subt.reverse():
                return False
            #reinitialize left and right subtree lists
            left_subt = []
            right_subt = []
        return True

I have run the algorithm with the following input: [1,2,2,3,4,4,3] ; it is the input that you see at the top of the page when you click on the link to Leetcode above.

It is returning false when it should be returning true. When I run the input on paper it seems to work, but unlike arrays/strings, I am not sure how to print node values at each stage. Any ideas how to do this, or can someone please outline where the code falls short?

Patrick Chong
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2 Answers2

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I am not sure how to print node values at each stage.

printing values in python is as easy as:

print(<something>)

where <something> is whatever you need to print

for debug purposes you can at any point in your code add for example:

print(node.val)

or

print(queue)
Dharman
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Almog-at-Nailo
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At least the following snippet will not work as you expected

for _ in range((level_length//2)+1):
    node = queue.popleft()
    left_subt.append(node.val)
    if node.left:
        queue.append(node.left)
    if node.right:
        queue.append(node.right)

Right at the 1st loop, at which, level_length == 2 (assume that both root.left & root.right are not None )

-> (level_length//2)+1 will return (1 + 1)

-> The 1st for statement will loop two times, and consume both root.left & root.right in your queue before the 2nd for statement starts

-> nothing's left for your right_subt

-> In the 2nd for statement, queue.popleft() will throw an exception due to the fact that you tried to 'pop from an empty deque'

Le Huy Hoang
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