191. Number of 1 Bits

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.

思路：愚蠢如我，一开始只想到把十进制转换成二进制然后数数。后来又想利用2^n-1的规律。

然后一看答案直接给跪。

n&n-1，就是这么简单。每次n&n-1都会去掉一个1。

tips: 关于uint32_t：uint8_t，uint16_t，uint32_t等都不是什么新的数据类型，它们只是使用typedef给类型起的别名，新瓶装老酒的把戏。

class Solution {
public:
int hammingWeight(uint32_t n) {
int res=0;
while(n){
n&=n-1;
res++;
}
return res;
}
};