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Consider the following Python code implementing the optimal backtracking solution for Combination Sum II. What is the final returned list when calling combinationSum2([1,2,2], 4)?

easy🧾 Code Trace Q12 of Q15

Subsets & Combinations - Combination Sum II (No Reuse, Duplicates)

Consider the following Python code implementing the optimal backtracking solution for Combination Sum II. What is the final returned list when calling combinationSum2([1,2,2], 4)?

A[[1,2,2]]

B[[1,2],[2]]

C[[1,2],[2,2]]

D[[1,2],[2,2],[1,2,2]]

Step-by-Step Solution

Step 1: Sort candidates and start backtracking
Sorted candidates: [1,2,2]. Start from index 0 with target=4.
Step 2: Explore combinations skipping duplicates at same level
Try 1 -> target=3, then try 2 at next index -> target=1, next 2 is duplicate at same level skipped. No further candidates fit. Backtrack and try 2 at index 1 -> target=2, next 2 at index 2 -> target=0, add [2,2]. Both [1,2,2] and [2,2] are valid unique combinations.
Final Answer:
Option C -> Option C
Quick Check:
Both [1,2,2] and [2,2] sum to 4 without reuse and duplicates skipped [OK]

Quick Trick: Duplicate skipping prevents repeated combinations [OK]

Common Mistakes:

MISTAKES

Including partial sums like [2]
Allowing reuse of elements
Not skipping duplicates at same recursion level

Trap Explanation:

PITFALL

Candidates may think [2,2] alone is invalid, but sum is 4 and no reuse is needed here.

Interviewer Note:

CONTEXT

Tests candidate's ability to mentally execute backtracking with duplicate skipping.

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