hdu2824 The Euler function O(n)求欧拉函数

hdu2824 The Euler function

O(n)求欧拉函数

#include <bits/stdc++.h> 
#define ll long long 
using namespace std ; 

const int N = 3000011 ; 
int l,r ; 
int prime[ 300011 ],phi[N] ;  
ll sum ; 
bool f[N] ; 

inline void Euler_init(int NN)  
{
    phi[ 1 ] = 1 ; f[ 1 ] = 1 ; 
    prime[ 0 ] = 0 ; 
    for(int i=2;i<=NN;i++) 
    {
        if( !f[ i ] ) prime[++prime[0]] = i,phi[ i ] = i-1 ; 
        for(int j=1;j<=prime[ 0 ] && i*prime[ j ]<=NN;j++) 
        {
            f[ i*prime[ j ] ] = 1 ; 
            if( i%prime[ j ] == 0 ) 
            {
                phi[ i*prime[ j ] ] = phi[ i ] * prime[ j ] ; 
                break ; 
            }
            else phi[ i*prime[ j ] ] = phi[ i ] * (prime[ j ]-1) ; 
        }
    }
}

int main() 
{
    Euler_init(3000000) ;   
    
    while(~scanf("%d%d",&l,&r)) 
    {
        sum = 0 ; 
        for(int i=l;i<=r;i++) 
            sum = sum + phi[ i ] ; 
        printf("%lld
",sum) ; 
    }    
    return 0 ; 
}