HDU 5901 Count primes

题目链接:

http://acm.hdu.edu.cn/showproblem.php?pid=5901

Problem Description

Easy question! Calculate how many primes between [1...n]!

 

 

Input

Each line contain one integer n(1 <= n <= 1e11).Process to end of file.

 

Output

For each case, output the number of primes in interval [1...n]

 

Sample Input

2

3

10

 

Sample Output

1

2

4

 

Hint:

题意：

比较简单，这里就不说了。 

题解:

自己开始的时候数据范围没有看清楚，以为是水题，结果就GG了。

网上大神的代码，自己好好学习学习。

代码：

#include <bits/stdc++.h>
#define ll long long
using namespace std;
ll f[340000],g[340000],n;
void init(){
    ll i,j,m;
    for(m=1;m*m<=n;++m)f[m]=n/m-1;
    for(i=1;i<=m;++i)g[i]=i-1;
    for(i=2;i<=m;++i){
        if(g[i]==g[i-1])continue;
        for(j=1;j<=min(m-1,n/i/i);++j){
            if(i*j<m)f[j]-=f[i*j]-g[i-1];
            else f[j]-=g[n/i/j]-g[i-1];
        }
        for(j=m;j>=i*i;--j)g[j]-=g[j/i]-g[i-1];
    }
}
int main(){
    while(scanf("%I64d",&n)!=EOF){
        init();
        cout<<f[1]<<endl;
    }
    return 0;
}