leetcode: (191) Number of 1 Bits

【Question】

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.
题目要求求出2进制中1的个数
方法一：可以通过逐位与1进行与，可以得到总的1个数；

class Solution {
public:
int hammingWeight(unsigned int n) {
int i;
int count=0;
for(int i=0;i<32;i++)
{
if ((n&1)==1) count++;
n>>=1;
}
return count;
}
};

方法一需要比较32次不够优化

方法二：通过比较n&n-1，可以将非1的位省去，如n=10000000000000000000000000， 进行n&n-1只需要一次元算，而方法一要32次

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