I Can’t Figure Out Why This Code Fails (and Neither Can ChatGPT)

Recently, I encountered an interesting problem while attempting to solve a LeetCode challenge that required checking whether a binary tree is fully symmetrical. Despite my best efforts, my initial implementation produced incorrect results for certain test cases. In this post, I’ll walk through the problem, my initial approach, the debugging process, and the eventual solution.

The Problem

The task was to determine if a binary tree is symmetric around its center. A symmetric tree is one where the left subtree is a mirror reflection of the right subtree. For example, the tree represented by the array [1, 2, 2, 3, 4, 4, 3] is symmetric.

The Original Code

Here’s the original code I wrote to solve the problem:

python def isSymmetric(root): if not root: return True

def isMirror(t1, t2):
    if not t1 and not t2:
        return True
    if not t1 or not t2:
        return False
    return (t1.val == t2.val) and isMirror(t1.right, t2.left) and isMirror(t1.left, t2.right)

return isMirror(root.left, root.right)

The Issue

When testing this code with the input [1, 2, 2, 3, 4, 4, 3], the function incorrectly returned false instead of the expected true. This was puzzling because the logic seemed sound at a glance.

The Debugging Journey

In my attempts to debug, I consulted various resources and even sought help from ChatGPT. Unfortunately, neither my own analysis nor the AI’s suggestions pinpointed the issue.

After further investigation, I realized that the problem might stem from the way the recursive function isMirror was structured. Specifically, I suspected that the conditional checks could be improperly aligned.

The Fix

After careful consideration and a bit of refactoring, I made a change that resolved the issue:

python def isSymmetric(root): if not root: return True

def isMirror(t1, t2):
    if not t1 and not t2:
        return True
    if not t1 or not t2:
        return False
    return (t1.val == t2.val) and isMirror(t1.left, t2.right) and isMirror(t1.right, t2.left)

return isMirror(root.left, root.right)

The key change was swapping the order of t1.left and t1.right in the recursive calls. This alteration ensures that the left subtree is correctly compared to the right subtree in a mirrored fashion.

Insights and Takeaways

Reflecting on this experience, a few important lessons stand out:

1. Code Readability: It’s crucial to maintain clear and explicit code. Indentation can be misleading, and using curly braces {} in languages like Python can enhance readability (though Python uses indentation, the concept applies in other languages).

2. Debugging: Sometimes, the issue is not always apparent, even with assistance from advanced tools like AI. A fresh perspective or a thorough review of the logic can often lead to the solution.

3. Community Help: Don’t hesitate to reach out to the programming community for assistance. Other developers can provide insights that you may have overlooked.

If anyone else has faced similar issues with binary tree problems or has insights into debugging recursive functions, I’d love to hear your thoughts!