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# 191. Number of 1 Bits

## Description

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 = 00000000000000000000000000001011Output:3Explanation:The input binary string00000000000000000000000000001011has a total of three '1' bits.

**Example 2:**

Input:n = 00000000000000000000000010000000Output:1Explanation:The input binary string00000000000000000000000010000000has a total of one '1' bit.

**Example 3:**

Input:n = 11111111111111111111111111111101Output:31Explanation:The input binary string11111111111111111111111111111101has 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?