poj 3904 Sky Code（莫比乌斯函数）

莫比乌斯函数，把所有的是i的倍数的数；num[i]如果大于四个就用C（4，num[i]）*mu[i]就是答案

#include <iostream>#include <stdio.h>#include <string.h>#define ll long longusing namespace std;const int MAX=11000;int mu[MAX],n,max1;int cnt[MAX];int num[MAX];void init(){  memset(mu,0,sizeof(mu));    mu[1]=1;    for(int i=1;i<MAX-10;i++)        for(int j=i+i;j<MAX-10;j+=i)        mu[j]-=mu[i];}long long solve(){    for(int i = 1; i <= max1; i++)        for(int j = i; j <= max1; j += i)            num[i] += cnt[j];    ll ans = 0;    for(int i = 1; i <= max1; i++)    {        int x = num[i];        if(x >= 4)            ans += (ll) mu[i] * x * (x - 1) * (x - 2) * (x - 3) / 24;    }    return ans;}int main(){    init();    while(~scanf("%d",&n))    {        max1=0;        memset(cnt,0,sizeof(cnt));        memset(num, 0, sizeof(num));        for(int i = 0; i < n; i++)        {            int tmp;            scanf("%d", &tmp);            cnt[tmp] ++;            max1 = max(max1, tmp);        }        if(n<4)printf("0\n");        else    printf("%lld\n",solve());    }    return 0;}