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Suppose you need to reverse bits of a 64-bit unsigned integer instead of 32-bit. Which modification to the optimal divide and conquer mask approach is correct?

hard🎤 Interviewer Follow-up Q15 of Q15

Bit Manipulation - Reverse Bits

Suppose you need to reverse bits of a 64-bit unsigned integer instead of 32-bit. Which modification to the optimal divide and conquer mask approach is correct?

AExtend masks to 64-bit constants and add an additional step shifting by 32 bits with proper masking.

BApply the 32-bit mask approach directly without changes; it works for any bit width.

CUse the brute force bit-by-bit reversal loop since masks only work for 32 bits.

DUse the same 32-bit masks and steps, but apply them twice to the lower and upper 32 bits separately.

Step-by-Step Solution

Solution:

Step 1: Understand mask size dependency
The masks and shifts are designed for 32-bit integers and must be extended for 64-bit integers.
Step 2: Modify masks and add step
Use 64-bit masks and add a final step shifting by 32 bits with masking to handle full 64-bit reversal.
Final Answer:
Option A -> Option A
Quick Check:
64-bit reversal requires 64-bit masks and an extra step [OK]

Quick Trick: 64-bit reversal needs 64-bit masks and extra shift step [OK]

Common Mistakes:

MISTAKES

Trying to reuse 32-bit masks directly
Splitting 64-bit into two 32-bit halves without merging
Assuming brute force is only option for 64-bit

Trap Explanation:

PITFALL

Many think 32-bit masks apply unchanged or brute force is necessary, but masks can be extended with an extra step.

Interviewer Note:

CONTEXT

Tests candidate's ability to generalize bit manipulation algorithms to larger word sizes.

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