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The following code attempts to solve Combination Sum III but contains a subtle bug. Identify the line causing incorrect results or inefficiency.

medium🐞 Bug Identification Q14 of Q15
Subsets & Combinations - Combination Sum III (K Numbers to N)
The following code attempts to solve Combination Sum III but contains a subtle bug. Identify the line causing incorrect results or inefficiency.
ALine appending comb directly to res without copying
BLine calculating min_sum for pruning
CLine popping last element from comb after recursion
DLine checking if total > n to prune early
Step-by-Step Solution

  1. Step 1: Identify how results are stored

    Appending comb directly stores a reference, so later modifications affect stored results.

  2. Step 2: Understand impact

    Must append a copy (comb[:]) to avoid all results being the same final list state.

  3. Final Answer:

    Option A -> Option A

  4. Quick Check:

    Appending reference causes all results to be identical after backtracking [OK]
Quick Trick: Always append a copy of combination, not the list reference [OK]
Common Mistakes:
MISTAKES
  • Appending reference instead of copy
  • Missing pruning conditions
Trap Explanation:
PITFALL
  • Appending reference looks correct but causes subtle bugs in output correctness.
Interviewer Note:
CONTEXT
  • Tests candidate's attention to detail in backtracking state management.
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