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The following code attempts to generate all unique subsets from an array with duplicates. Which line contains a subtle bug that causes incorrect or duplicate subsets?

medium🐞 Bug Identification Q14 of Q15

Subsets & Combinations - Subsets II (With Duplicates)

The following code attempts to generate all unique subsets from an array with duplicates. Which line contains a subtle bug that causes incorrect or duplicate subsets?

ALine 9: if i > 0 and nums[i] == nums[i - 1]: s = 0

BLine 13: res.append(res[j] + [nums[i]])

CLine 11: start = len(res)

DLine 6: nums.sort()

Step-by-Step Solution

Solution:

Step 1: Understand the role of variable s
Variable s determines the start index for adding new subsets. For duplicates, s should be set to the previous start to avoid duplicates.
Step 2: Identify the bug in line 9
Setting s = 0 for duplicates causes subsets to be appended from the beginning, generating duplicate subsets. The correct logic is to set s = start (previous length) to skip duplicates only at the same recursion level.
Final Answer:
Option A -> Option A
Quick Check:
Incorrect s causes duplicate subsets [OK]

Quick Trick: Duplicate skipping must use previous start index [OK]

Common Mistakes:

MISTAKES

Setting s=0 always for duplicates
Not sorting input before processing
Appending without proper start index

Trap Explanation:

PITFALL

Setting s=0 looks plausible but breaks duplicate skipping logic subtly.

Interviewer Note:

CONTEXT

Tests ability to spot subtle off-by-one or indexing bugs in backtracking.

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