二进制数中1的个数

a：除法 b：移位 c：减法（一下子不好想到） d：穷举 e：穷举（直接查表，速度快）

#include <stdio.h>

int Count1(char v);
int Count2(char v);
int Count3(char v);
int Count4(char v);
int Count5(char v);

int
main(void)
{
        char v = 0x04;

        printf("count1 = %d\n", Count1(v));
        printf("count2 = %d\n", Count2(v));
        printf("count3 = %d\n", Count3(v));
        printf("count4 = %d\n", Count4(v));
        printf("count5 = %d\n", Count5(v));

        return (0);
}

/*
 * a
 */
int
Count1(char v)
{
        int num = 0;
        while (v) {
                if (v % 2 == 1) {
                        ++num;
                }
                v /= 2;
        }
        return (num);
}

/*
 * b
 */
int
Count2(char v)
{
        int num = 0;
        while (v) {
                num += v & 0x01;
                v >>= 1;
        }
        return (num);
}

/*
 * c
 */
int
Count3(char v)
{
        int num = 0;
        while (v) {
                v &= (v-1);
                ++num;
        }
        return (num);
}

/*
 * d
 */
int
Count4(char v)
{
        int num = 0;
        switch (v) {
                case 0x00: num = 0; break;
                //...
        }
        return (num);
}

/*
 * e
 */
int
Count5(char v)
{
        int countTable[256] =
                {
                //...
                };
        return (0);
}