Defuse the Bomb

Defuse the Bomb

What is this problem about?

The Defuse the Bomb interview question asks you to decrypt a circular array of numbers based on a key k.

• If k > 0, replace each element with the sum of the next k numbers.
• If k < 0, replace each element with the sum of the previous |k| numbers.
• If k = 0, replace each element with 0. Since the array is circular, the "next" or "previous" elements wrap around the boundaries. This Defuse the Bomb coding problem is a test of circular indexing and window summation.

Why is this asked in interviews?

Companies like Microsoft and Amazon use this to evaluate a candidate's ability to handle array wrap-around logic. It tests whether you can use the modulo operator effectively or if you prefer to "double" the array to simplify indices. It also assesses your ability to optimize repeated summations using a sliding window approach.

Algorithmic pattern used

This follows the Array, Sliding Window interview pattern.

1. k=0: Return an array of all zeros.
2. Sliding Window: Instead of re-calculating the sum for every index (O(N * K)), maintain a running sum of |k| elements.
3. Circular Indexing: Use (i + j) % n to find indices for the window.
4. Process: Initialize the sum for the first window, then shift it for each subsequent element by adding the new element entering the window and subtracting the one leaving it.

Example explanation

nums = [5, 7, 1, 4], k = 3.

1. Initialize window sum for index 0 (next 3): 7+1+4 = 12.
2. Index 1: New element entering window is nums[(1+3)%4] = 5. Element leaving is nums[(0+1)%4] = 7. New sum: 12 + 5 - 7 = 10.
3. Index 2: New element 7, leaving 1. New sum: 10 + 7 - 1 = 16.
4. Index 3: New element 1, leaving 4. New sum: 16 + 1 - 4 = 13. Result: [12, 10, 16, 13].

Common mistakes candidates make

• Inefficient summation: Using nested loops to calculate the sum for every index, leading to poor performance on large inputs.
• k < 0 logic: Failing to correctly identify the starting range for the sum when k is negative.
• Modulo confusion: Incorrectly applying the modulo operator, especially with negative offsets.

Interview preparation tip

Whenever a problem involves "circular arrays," think about the modulo operator: index = (current + offset) % length. If the offset can be negative, use (current + offset + length) % length to ensure a positive result.