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

For example, the 32-bit integer ’11' has binary representation 00000000000000000000000000001011, so the function should return 3.

n & (n - 1)能够将最低的位1置为0 如00001011 n 00001011 n-1 00001010 & 00001010 有几次上述操作，就有几个1