BZOJ 4146: [AMPPZ2014]Divisors

Description

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

Solution

欧拉筛.

一个欧拉筛统计一下...没了...相同的数是(c_i imes (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
",ans);
    return 0;
}