HDOJ 2824 The Euler function （欧拉函数打表）

The Euler function

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 4658    Accepted Submission(s): 1953

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到b之间的欧拉数之和

思路：暴力肯定超时，300w的数据量太大了，需要技巧打表

ac代码：

#include<stdio.h>#include<string.h>#include<iostream>#include<algorithm>#define MAXN 3000001#define MAX(a,b) a>b?a:b#define LL long long#define fab(a) ((a)>0?(a):-(a))using namespace std;LL ans[MAXN];void db(){memset(ans,0,sizeof(ans));ans[1]=1;int i,j;for(i=2;i<=MAXN;i++){if(!ans[i]){for(j=i;j<=MAXN;j+=i){if(!ans[j])ans[j]=j;ans[j]=ans[j]/i*(i-1);    }}}}int main(){int i,a,b;db();while(scanf("%d%d",&a,&b)!=EOF){LL sum=0;for(i=a;i<=b;i++)sum+=ans[i];printf("%lld\n",sum);}return 0;}