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Examine the following buggy code snippet from the trie-based solution. Which line contains the subtle bug that causes incorrect counts for some puzzles?

medium🐞 Bug Identification Q14 of Q15
Subsets & Combinations - Number of Valid Words for Each Puzzle
Examine the following buggy code snippet from the trie-based solution. Which line contains the subtle bug that causes incorrect counts for some puzzles?
ALine where node.count is incremented
BLine missing filtering words with more than 7 unique letters before insertion
CLine checking if bit is set in mask during insertion
DLine checking if puzzle_mask contains child bit during dfs
Step-by-Step Solution

  1. Step 1: Identify filtering step

    The code inserts all words regardless of unique letter count, causing large trie and slow traversal.

  2. Step 2: Understand impact

    Words with >7 unique letters cannot match puzzles (7 letters max), so filtering them out is necessary for correctness and efficiency.

  3. Final Answer:

    Option B -> Option B

  4. Quick Check:

    Filtering missing leads to incorrect counts and performance issues [OK]
Quick Trick: Filter words with >7 unique letters before insertion [OK]
Common Mistakes:
MISTAKES
  • Not filtering large words
  • Incorrect bit checks
  • Misplaced count increments
Trap Explanation:
PITFALL
  • Code looks correct but missing filtering causes subtle bugs and inefficiency.
Interviewer Note:
CONTEXT
  • Tests candidate's attention to subtle pruning needed for correctness and performance.
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