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Examine the following buggy backtracking code for generating combinations of size k from 1 to n. Which line contains the subtle bug that causes incorrect or duplicate results?

medium🐞 Bug Identification Q14 of Q15

Subsets & Combinations - Combinations (Choose K from N)

Examine the following buggy backtracking code for generating combinations of size k from 1 to n. Which line contains the subtle bug that causes incorrect or duplicate results?

ALine where for loop iterates from start to n + 1

BLine where recursion starts from 1 instead of 0

CLine where path.pop() is called after recursion

DLine where path is appended directly to result without copying

Step-by-Step Solution

Step 1: Identify how results are stored
Appending path directly stores a reference, so all entries in result point to the same list object.
Step 2: Understand consequence
All combinations in result become identical after backtracking modifies path, causing duplicates or incorrect results.
Final Answer:
Option D -> Option D
Quick Check:
Appending a copy (path[:]) fixes the bug [OK]

Quick Trick: Always append a copy of path, not the path itself [OK]

Common Mistakes:

MISTAKES

Forgetting to copy path before appending to result

Trap Explanation:

PITFALL

Appending path directly looks correct but causes subtle reference sharing bugs.

Interviewer Note:

CONTEXT

Tests attention to detail and understanding of mutable references in recursion.

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