C语言 求一个整数存储在内存中的二进制中1的个数。

#include <stdio.h>
int statistics1(unsigned int num)
{
    
    
	int count = 0;
	while (num)
	{
    
    
		if (num % 2 == 1)
		{
    
    
			count++;
		}
		num /= 2;
	}
	return count;
}

int statistics2(int num)
{
    
    
	int count = 0;
	for (int i = 0; i < 32; i++)
	{
    
    
		if (((num >> i) & 1) == 1)
			count++;
	}
	return count;
}
int statistics3(int num)//效率最高的方法
{
    
    
	int count = 0;
	while (num)
	{
    
    							//例如：13
		num = num & (num - 1);  //1101&1100==1100
								//1100&1011==1000
								//1000&0111==0000
		//每进行一次按位与操作 num二进制就少一位1.
		count++;
	}
	return count;
}
int main()
{
    
    
	int num = -1;
	int b = statistics1(num);
	int c = statistics2(num);
	int d = statistics3(num);
	printf("%d %d %d", b, c, d);//32 32 32
}