HDU 2824 欧拉函数筛法

存模板

    #include <iostream>  
    #include <stdio.h>  
    #include <memory.h>  
    #include <math.h>  
    using namespace std;  
      
    const int N = 3000005;  
      
    int prime[N];  
    int phi[N];  
    bool is_prime[N];  
      
    void get_phi()  
    {  
        int i, j, k;  
        k = 0;  
        for(i = 2; i < N; i++)  
        {  
            if(is_prime[i] == false)  
            {  
                prime[k++] = i;  
                phi[i] = i-1;  
            }  
            for(j = 0; j<k && i*prime[j]<N; j++)  
            {  
                is_prime[ i*prime[j] ] = true;  
                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()  
    {  
        long long sum;  
        int i, x, y;  
        get_phi();  
        while(scanf("%d %d", &x, &y) != EOF)  
        {  
            sum = 0;  
            for(i = x; i <= y; i++)  
            {  
                sum += phi[i];  
            }  
            printf("%I64d\n", sum);  
        }  
      
        return 0;  
    }  

 乘法逆元 ON

inv[1] = inv[0] = 1;
for(int i = 2;i < N;i++)
inv[i] = inv[mod%i]*(mod-mod/i)%mod;