NumberOf1Bits.java

// Source : https://leetcode.com/problems/number-of-1-bits/
// Author : zhoutianbin
// Date   : 2021-03-22

/***************************************************************************************************** 
 *
 * 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?
 ******************************************************************************************************/

package NumberOf1Bits;

public class NumberOf1Bits {
    public int hammingWeight(int n) {
        int num = 0;
        while (n != 0) {
            num++;
            n = n & (n - 1);
        }
        return num;
    }

    public int hammingWeight2(int n) {
        int num = 0;
        int t = 1;
        while (n != 0) {
            num = (n & t) == 1 ? num + 1 : num;
            n >>>= 1;
        }
        return num;
    }

    public int hammingWeight3(int n) {
        int num = 0;
        int t = 1;
        for (int i = 0; i < 32; i++) {
            num = (n & t) == t ? num + 1 : num;
            t <<= 1;
        }
        return num;
    }
}