Скачать или смотреть How to Fix Recursive Tree Insertion Only Creating Root Node in Python

Learn how to solve the issue of recursive tree insertion in Python that results in only creating the root node, and transform your data structure!
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This video is based on the question https://stackoverflow.com/q/67428592/ asked by the user 'David Sousa' ( https://stackoverflow.com/u/13765972/ ) and on the answer https://stackoverflow.com/a/67450373/ provided by the user 'trincot' ( https://stackoverflow.com/u/5459839/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem: Recursive Tree Insertion

If you've been working with trees in Python, you might have encountered a frustrating issue: your recursive tree insertion function only creates a root node. In this post, we’ll unravel why this happens and guide you through a clear and structured solution.

The Scenario

You have a function designed to insert values into a binary tree based on the maximum value from a provided vector (or array). The idea is to:

Find the maximum value and create a root node.

Divide the vector into two parts: left of the maximum value and right of it.

Recursively assign these parts to the left and right children of the root node.

However, upon running the code, you realize that only the root node is filled in, and the left child remains None. Why does this happen?

Root of the Problem

The main issue arises from how Python handles variable assignments. In particular:

When calling the helper method _insert, the second argument (the left or right child node) is always None.

In Python, arguments are passed by value (specifically references to objects), not by reference. This means that while you can create new objects inside a function, any modifications you make to the parameters will not affect the caller's variables.

As a result, any new nodes created in the helper function are not being assigned to the left or right properties of your current node.

The Solution: Return the Node

To fix this issue, you need to adjust how the _insert function operates. Instead of passing the node as an argument, the function should simply return the newly created node. Here's how to achieve that.

Step-by-Step Changes

Modify the _insert Function:

Remove the second argument from the function.

Return the created node.

Here's the revised version:

[[See Video to Reveal this Text or Code Snippet]]

Update the Main insert Method:

Delegate the responsibility to assign the root to _insert by directly calling it without a second parameter.

Refactor the insert method like this:

[[See Video to Reveal this Text or Code Snippet]]

Example Run

With these changes, you can now run the program:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By understanding the concept of variable passing in Python and restructuring your function to return new nodes, you can effectively build a binary tree that reflects all the necessary values instead of just the root.

This method not only solves the immediate problem but also enhances the readability and efficiency of your code. Now, you can confidently work with recursive functions and tree structures in Python!

Feel free to explore more data structure-related topics and enhance your programming skills further!