Write a function that takes an unsigned integer and returns the number of '1' bits it has (also known as the Hamming weight).
Example:
Input: n = 00000000000000000000000000001011
Output: 3
Explanation: The input binary string 00000000000000000000000000001011 has a total of three '1' bits.
Constraints:
- The input must be a binary string of length
32