Count Set Bits / Reverse Bits

Question

## Problem

Count the number of 1-bits in an integer, and separately reverse the bits of a 32-bit unsigned integer.

## Example
```
countBits(0b1011) -> 3; reverse(0b...0001) -> 0b1000...0
```

## Constraints
- 32-bit unsigned

## Approach
Brian Kernighan's `n &= n-1` for counting; bit-by-bit swap or lookup table for reverse.