HDU 2824 The Euler function （欧拉函数水题）

The Euler function

 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) 

Input
There are several test cases. Each line has two integers a, b（2 < a < b < 3000000）
Output
Output the result of (a)+ (a+1)+….+ (b)
Sample Input

3 100

Sample Output

3042

裸的欧拉函数题

一开始想预处理处前缀和直接输出，结果内存超了，然后一个一个暴力加起来就过了。。

code：

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn = 3000010;
int phi[maxn];
void Euler(){
    phi[1] = 1;
    for(int i = 2; i < maxn; i++){
        phi[i] = i;
    }
    for(int i = 2; i < maxn; i++){
        if(phi[i] == i){
            for(int j = i; j < maxn; j += i){
                phi[j] = phi[j] / i * (i - 1);
            }
        }
    }
}
int main(){
    Euler();
    int a,b;
    while(scanf("%d%d",&a,&b)!= EOF){
        ll ans = 0;
        for(int i = a; i <= b; i++){
            ans += (ll)phi[i];
        }
        printf("%lld\n",ans);
    }
    return 0;
}