BZOJ 4146: [AMPPZ2014]Divisors

Description

给定一个序列\(a_1,a_2,...,a_n\)。求满足\(i\neq j\)且\(a_i \mid a_j\)的二元组\((i,j)\)的个数,\(n,a_i\leqslant 2\times 10^6\)。

Solution

欧拉筛.

一个欧拉筛统计一下...没了...相同的数是\(c_i\times (c_i-1)\)

Code

/**************************************************************
    Problem: 4146
    User: BeiYu
    Language: C++
    Result: Accepted
    Time:7984 ms
    Memory:9104 kb
****************************************************************/
 
#include <bits/stdc++.h>
using namespace std;
 
typedef long long ll;
const int N = 2000500;
 
inline int in(int x=0,char s=getchar()) { while(s>'9'||s<'0')s=getchar();
    while(s>='0'&&s<='9')x=x*10+s-'0',s=getchar();return x; }
 
int n;
int a[N];
ll ans;
 
int main() {
    n=in();
    for(int i=1,x;i<=n;i++)  x=in(),a[x]++;
    for(int i=1;i<=2000000;i++) {
        ans+=1ll*a[i]*(a[i]-1);
        for(int j=i+i;j<=2000000;j+=i) ans+=1ll*a[i]*a[j];
    }printf("%lld\n",ans);
    return 0;
}