191. Number of 1 Bits

问题
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.

位运算，[n-1]的二进制数字为[n]的二进制数字退一位。
二者的与运算结果相当于二进制下[n]减少最右侧的1。
例如0100100的0100011
两者的与运算结果为0100000。相当于减少了一位1。
计算循环次数就可以得出1的总数。

public class Solution {
    // you need to treat n as an unsigned value
    public int hammingWeight(int n) {
        int count = 0;
        while(n != 0){
            n = (n & (n-1));
            count++;
        }
        return count;
    }
}