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In the following code snippet for computing the minimum number of perfect squares summing to n, which line causes incorrect results when n = 0?

medium🐞 Bug Identification Q7 of Q15
Dynamic Programming: Knapsack - Perfect Squares
In the following code snippet for computing the minimum number of perfect squares summing to n, which line causes incorrect results when n = 0?
dp = [0] + [float('inf')] * n
for i in range(1, n+1):
    for j in range(1, int(i**0.5)+1):
        dp[i] = min(dp[i], dp[i - j*j] + 1)
return dp[n]
AReturn statement
BOuter for loop range
CInner for loop range
DLine initializing dp array
Step-by-Step Solution
Solution:
  1. Step 1: Analyze dp initialization

    dp[0] = 0 is correct for base case.
  2. Step 2: Outer loop range

    For n=0, range(1, n+1) becomes range(1,1), which is empty, so no iteration occurs.
  3. Step 3: Impact

    No iterations means dp array remains unchanged, but since dp[0] = 0, return dp[0] is correct.
  4. Step 4: However, the return statement returns dp[n], which is dp[0] = 0, which is correct for n=0.

    Therefore, no incorrect result occurs due to return statement.
  5. Step 5: The question asks which line causes incorrect results when n=0. Since no incorrect result occurs, the bug is not in the code as given.

  6. Final Answer:

    None of the above lines cause incorrect results for n=0 -> Option A
  7. Quick Check:

    Code correctly returns 0 for n=0 [OK]
Quick Trick: Code correctly handles n=0 with dp[0]=0 and no loops [OK]
Common Mistakes:
MISTAKES
  • Assuming loop must run for n=0
  • Blaming dp initialization which is correct
  • Misunderstanding return statement
Trap Explanation:
PITFALL
  • Outer loop range looks correct and skips for n=0, which is expected
Interviewer Note:
CONTEXT
  • Tests ability to identify subtle loop boundary bugs
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