Given a positive integer n, write a function that returns the number of set bits in its binary representation (also known as the [Hamming weight](https://en.wikipedia.org/wiki/Hamming_weight)).
Example 1:
Input: n = 11
Output: 3
Explanation: The input binary string 1011 has a total of three set bits.
Example 2:
Input: n = 128
Output: 1
Explanation: The input binary string 10000000 has a total of one set bit.
Example 3:
Input: n = 2147483645
Output: 30
Explanation: The input binary string 01111111111111111111111111111101 has a total of thirty set bits.
1 <= n <= 2^31 - 1