HDU 2824 The Euler function

Problem Description
The Euler function phi is an important kind of function in number theory, (n) represents the amount of the numbers which are smaller than n and coprime to n, and this function has a lot of beautiful characteristics. Here comes a very easy question: suppose you are given a, b, try to calculate (a)+ (a+1)+….+ (b)

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【题目分析】
只需要处理出来300w以内的欧拉函数的值。用前缀和算一下区间的值就可以了。注：要用longlong。

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【代码】

#include <cstdio>long long n=3000000,phi[3000001],p[1500001],top=0;bool ma[3000001];void init(){     phi[1]=1;     for (int i=2;i<=n;++i)     {         if (!ma[i]) ma[i]=true,p[++top]=i,phi[i]=i-1;         for (int j=1;j<=top&&i*p[j]<=n;++j)         {             ma[i*p[j]]=true;             if (i%p[j]==0){phi[i*p[j]]=phi[i]*p[j];break;}             else phi[i*p[j]]=(p[j]-1)*phi[i];         }     }}int main(){    int l,r; init();    for (int i=1;i<=n;++i) phi[i]+=phi[i-1];    while (~scanf("%d%d",&l,&r)) printf("%lld\n",phi[r]-phi[l-1]);}