Dynamic Programming: Knapsack - Target Sum
The following code attempts to solve the Target Sum problem using bottom-up DP with offset indexing. Which line contains a subtle bug that causes incorrect results on inputs containing zero?
The following code attempts to solve the Target Sum problem using bottom-up DP with offset indexing. Which line contains a subtle bug that causes incorrect results on inputs containing zero?
medium🐞 Bug Identification Q14 of Q15
Step-by-Step Solution
Step 1: Identify DP update method
The code updates dp in-place during iteration over sums, causing double counting when num=0 or overlapping states.Step 2: Understand impact on zero values
Zero does not change sum, so updating dp in-place doubles counts incorrectly. Using a separate next_dp array avoids this.Final Answer:
Option A -> Option AQuick Check:
In-place updates cause state reuse within iteration, breaking correctness [OK]
Quick Trick: In-place dp updates cause double counting with zero values [OK]
Common Mistakes:
MISTAKES- Not using a separate dp array for next iteration
Trap Explanation:
PITFALL- In-place updates look efficient but break correctness for zero or overlapping sums.
Interviewer Note:
CONTEXT- Tests if candidate spots subtle DP update bugs affecting correctness.
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