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Consider the following buggy code for generating subsets. Which line contains the subtle bug that causes all subsets in res to be the same list instance?

medium🐞 Bug Identification Q14 of Q15

Bit Manipulation - Subsets Using Bitmask

Consider the following buggy code for generating subsets. Which line contains the subtle bug that causes all subsets in res to be the same list instance?

ALine 4: def backtrack(start, path):

BLine 7: path.append(nums[i])

CLine 5: res.append(path)

DLine 9: path.pop()

Step-by-Step Solution

Solution:

Step 1: Identify how subsets are added to results
Appending path directly adds a reference, not a copy, so all entries point to the same list object.
Step 2: Fix by appending a copy of path
Using path[:] or list(path) creates a snapshot copy, preserving each subset uniquely.
Final Answer:
Option C -> Option C
Quick Check:
Appending reference causes all subsets to be identical [OK]

Quick Trick: Always append a copy of the current path [OK]

Common Mistakes:

MISTAKES

Appending mutable list reference instead of a copy

Trap Explanation:

PITFALL

Appending the list reference looks correct but causes all results to share the same list object.

Interviewer Note:

CONTEXT

Tests understanding of mutable references and backtracking correctness.

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