191. Number of 1 Bits

1. Description

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

2. 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 above, the input represents the signed integer. -3.

3. Follow Up

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

4. Example

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.

5. Constraints

• The input must be a binary string of length 32

6. Solutions

My Accepted Solution

Time complexity: O(logn)
Space complexity: O(1)

class Solution {
public:
    int hammingWeight(int n) {
        int count = 0;
        for (int i = n; i > 0; ++count) {
            i &= i - 1;
        }

        return count;
    }
};