hdu-2824 The Euler function(欧拉函数)

题目链接：

The Euler function

Time Limit: 2000/1000 MS (Java/Others)   

 Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 4987    Accepted Submission(s): 2098

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)

 

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

题意：求[a,b]的欧拉函数和;

AC代码：

#include <iostream>
#include <cstring>
#include <cstdio>
#include <algorithm>
using namespace std;
const int N=3e6+3;
int phi[N];
int get_phi()
{
    for(int i=2;i<N;i++)
    {
        if(!phi[i])
        {
            for(int j=i;j<N;j+=i)
            {
                if(!phi[j])phi[j]=j;
                phi[j]=phi[j]/i*(i-1);
            }
        }
    }
}
int main()
{
    get_phi();
    int a,b;
    while(scanf("%d%d",&a,&b)!=EOF)
    {
        long long ans=0;
        for(int i=a;i<=b;i++)
        {
            ans+=(long long)phi[i];
        }
        cout<<ans<<"
";
    }
    return 0;
}