0191. Number of 1 Bits

https://leetcode.com/problems/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 = 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?

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