C++ 9行代码搞定该问题

用iostream不用namespace显得我很抠门，所以把它换成了stdio.h。这样看起来就正常多了~
至于return 0嘛~

#include <stdio.h>
int n, ans, book[23] = {1,0,0,0,1,0,1,0,2,1,0,0,0,0,0,0,0,1,2,0,1,0,0};
char tmp[9];
int main() {
    scanf("%d", &n);
    sprintf(tmp, "%X", n);
    for (auto& i : tmp) ans += book[i - '0'];
    printf("%d", ans);
}

10行正经的代码

#include <stdio.h>
int n, ans, book[16] = {1,0,0,0,1,0,1,0,2,1,1,2,0,1,0,0};
int main() {
    scanf("%d", &n);
    do {
        ans += book[n & 0xf];
        n >>= 4;
    } while(n);
    printf("%d", ans);
}