Скачать или смотреть How to Fix Your Recursive Function: Ensure It Returns the Last Value of a Linked List

Struggling with a recursive function that doesn't return the expected int value in Python? Discover the common mistake in handling recursive calls and learn how to fix it step by step.
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This video is based on the question https://stackoverflow.com/q/64961362/ asked by the user 'mesunmoon' ( https://stackoverflow.com/u/14277994/ ) and on the answer https://stackoverflow.com/a/64961412/ provided by the user 'Red' ( https://stackoverflow.com/u/13552470/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem: Recursive Function Not Returning Values in Python

If you're working with linked lists in Python, you may have come across a situation where your recursive function fails to return the expected integer value. This can be frustrating, especially when the function appears to operate correctly but doesn't yield the correct results. In this guide, we’ll explore a specific case where a last() function is designed to return the last integer value of a singly linked list but isn't functioning as intended.

The Node Class

Before diving into the solution, let’s first understand the Node class that represents each item in our singly linked list. The class has two attributes: data, which holds an integer, and next, which is a reference to the next Node in the list or None if it’s the last node.

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Now, let's take a look at the problematic function. This function is intended to return the last integer value of the linked list.

The Problematic Function

Here's where the confusion arises. The function is structured to traverse the linked list recursively, and it does print the intended output; however, it fails to return the value properly.

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The Issue

The main issue lies in the recursive call last(head.next). Since there is no return statement before this call, the function eventually returns None instead of the last integer value (310, in the provided example) from the linked list.

The Solution

To fix this problem, you need to ensure that the return value of the recursive call is properly returned to the caller. Here’s the revised version of the last() function:

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Key Changes Made

Removed unnecessary else statement since the return statements handle flow control adequately.

Added return in front of the last(head.next) call to ensure that the last value is correctly returned up the call stack.

Conclusion

By making these adjustments, your function should now properly return the last integer value of the linked list. When revising recursive functions, always remember the importance of return statements. They not only provide the expected output but also carry the value back through each layer of recursion.

Now, when you run your function, you can expect it to return 310 as intended, thus resolving the issue you faced. Happy coding!