Count primes

Time Limit: 12000/6000 MS (Java/Others)    Memory Limit: 65536/65536 K (Java/Others)
Total Submission(s): 521    Accepted Submission(s): 221

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

2310

 

Sample Output

124

 

Source

2016 ACM/ICPC Asia Regional Shenyang Online

题意不说了，很简单。

下面是网上大神的代码，表示现在自己也是看不懂啊。（以后弄懂了再补充）

#include <bits/stdc++.h>#define ll long longusing 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;        ll x=min(m-1,n/i/i);        for(j=1; j<=x; ++j)        {            if(i*j<m)f[j]-=f[i*j]-g[i-1];            else f[j]-=g[n/i/j]-g[i-1];        }        x=i*i;        for(j=m; j>=x; --j)g[j]-=g[j/i]-g[i-1];    }}int main(){    while(scanf("%I64d",&n)!=EOF)    {        init();        printf("%I64d\n",f[1]);    }    return 0;}