191. Number of 1 Bits #

Problem Statement: #

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?

Solution: #

public int hammingWeight(int n) {
  int count = 0,
      mask = 1;
  for (int i = 0; i < 32; i++) {
    if ((n & mask) != 0)
      count++;
    mask <<= 1;
  }
  return count;
}

... #