Fix demos showing "no output" by capturing return values from demo functions

[Copilot]

18 demos (primarily LeetCode problems) returned output strings instead of printing them. The run_demo() function only captured stdout, so these demos appeared to produce no output in the UI.

Changes

flask_app/app.py

• Modified run_demo() to capture both stdout and return values
• Falls back to return value when stdout is empty

buf = io.StringIO()
with contextlib.redirect_stdout(buf):
    result = demo_fn()

output = buf.getvalue()
if not output and result is not None:
    output = str(result)

flask_app/templates/demo.html

• Changed template condition from {% if output %} to {% if output is not none %}
• Distinguishes between empty string (show message) and None (show nothing)

Impact

Fixes output display for 18 demos across:

• interview_workbook.leetcode.binary_search/* (7 demos)
• interview_workbook.leetcode.linked_list/* (2 demos)
• interview_workbook.leetcode.two_pointers/* (8 demos)
• interview_workbook.leetcode._audit

Screenshot

"Add Two Numbers" demo now shows output "243 + 564 -> 7->0->8" instead of "no output"

Original prompt

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This section details on the original issue you should resolve

<issue_title>When you click run there are a number of demos that show "no output"</issue_title>
<issue_description>When you click run there are a number of demos that show "no output"</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes When you click run there are a number of demos that show "no output" #8

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[Copilot]

Pull Request Overview

This PR fixes an issue where 18 demos (primarily LeetCode problems) were returning output strings instead of printing them, causing the UI to display "no output" because run_demo() only captured stdout.

Key Changes:

• Modified run_demo() to capture and use return values when stdout is empty
• Updated the demo template to distinguish between "not run" (None), "run with no output" (empty string), and "run with output" (non-empty string)

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
flask_app/app.py	Enhanced run_demo() to capture both stdout and return values, falling back to return value conversion when stdout is empty
flask_app/templates/demo.html	Updated output section to use is not none check and display appropriate messages for empty vs. no output scenarios

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After thorough review of the changed code, I found no issues within the modified regions. The implementation correctly:

• Captures both stdout and return values from demo functions
• Prioritizes stdout over return values (as intended by design)
• Properly handles edge cases like falsy return values (0, False, empty collections)
• Uses appropriate template logic to distinguish between different output states

The changes successfully address the reported issue and follow sound engineering practices.

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