Identify the line containing the subtle bug that can cause incorrect results or infinite recursion.
medium🐞 Bug Identification Q14 of Q15
Subsets & Combinations - Matchsticks to Square
The following code attempts to solve the Matchsticks to Square problem using backtracking with bitmask and memoization. Identify the line containing the subtle bug that can cause incorrect results or infinite recursion.
ALine with 'if total % 4 != 0: return false' (no divisibility check)
BLine with 'matchsticks.sort(reverse=true)' (missing sorting causes inefficiency)
CLine with 'memo[(used_mask, curr_sum, sides_formed)] = false' (incorrect memoization)
DLine with recursive call 'backtrack(next_mask, next_sum, sides_formed)' missing return statement
Step-by-Step Solution
Solution:
Step 1: Identify missing return in recursion
The recursive call without return means the function ignores successful paths, causing incorrect results or infinite recursion.
Step 2: Confirm other lines are correct
Divisibility check is present, sorting is done, and memoization line is correct for caching failures.
Final Answer:
Option D -> Option D
Quick Check:
Missing return in recursive call breaks backtracking correctness [OK]
Quick Trick:Missing return in recursive call breaks correctness [OK]
Common Mistakes:
MISTAKES
Forgetting to return recursive call result
Skipping divisibility check
Not sorting before backtracking
Trap Explanation:
PITFALL
The missing return looks like a minor omission but causes major logical failure; other lines seem suspicious but are correct.
Interviewer Note:
CONTEXT
Tests attention to detail and understanding of backtracking recursion correctness.
Master "Matchsticks to Square" in Subsets & Combinations
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