Write a function that takes an unsigned integer and returns the number of '1' bits it has (also known as the Hamming weight).

Note:

- Note that in some languages, such as Java, there is no unsigned integer type. In this case, the input will be given as a signed integer type. It should not affect your implementation, as the integer's internal binary representation is the same, whether it is signed or unsigned.
- In Java, the compiler represents the signed integers using 2's complement notation. Therefore, in Example 3, the input represents the signed integer. `-3`.

Example 1:

    Input: n = 00000000000000000000000000001011
    Output: 3
    Explanation: The input binary string 00000000000000000000000000001011 has a total of three '1' bits.

Example 2:

    Input: n = 00000000000000000000000010000000
    Output: 1
    Explanation: The input binary string 00000000000000000000000010000000 has a total of one '1' bit.

Example 3:

    Input: n = 11111111111111111111111111111101
    Output: 31
    Explanation: The input binary string 11111111111111111111111111111101 has a total of thirty one '1' bits.

Constraints:

- The input must be a binary string of length `32`.

Follow up: If this function is called many times, how would you optimize it?