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Consider the following buggy code for lastStoneWeightII. Which line contains the subtle bug that causes incorrect results on some inputs?

medium🐞 Bug Identification Q14 of Q15
Dynamic Programming: Knapsack - Last Stone Weight II
Consider the following buggy code for lastStoneWeightII. Which line contains the subtle bug that causes incorrect results on some inputs?
ALine 4: dp[0] = true is missing
BLine 7: Outer loop over stones
CLine 8: Inner loop iterates backwards
DLine 10: Return statement calculation
Step-by-Step Solution

  1. Step 1: Identify dp initialization

    dp[0] must be true to represent sum zero achievable with no stones; missing this means no sums are marked achievable.

  2. Step 2: Consequence of missing dp[0] = true

    Without dp[0] = true, dp array remains false, so no sums can be formed, causing the function to fail to find any valid partition.

  3. Final Answer:

    Option A -> Option A

  4. Quick Check:

    Initializing dp[0] is critical base case for subset sum DP [OK]
Quick Trick: dp[0] = true is base case for subset sums [OK]
Common Mistakes:
MISTAKES
  • Forgetting dp[0] initialization
  • Iterating forward in inner loop causing double counting
  • Miscomputing return value
Trap Explanation:
PITFALL
  • Missing dp[0] = true looks like a minor omission but breaks all achievable sums logic.
Interviewer Note:
CONTEXT
  • Tests if candidate knows DP base cases and their importance in subset sum problems.
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