L - Prime Number（求n内质数的个数）

Description

Write a program which reads an integer n and prints the number of prime numbers which are less than or equal to n. A prime number is a natural number which has exactly two distinct natural number divisors: 1 and itself. For example, the first four prime numbers are: 2, 3, 5 and 7.

Input

Input consists of several datasets. Each dataset has an integer n (1 ≤ n ≤ 999,999) in a line.

The number of datasets is less than or equal to 30.

Output

For each dataset, prints the number of prime numbers.

Sample Input

Output for the Sample Input

4

2

5
解题思路：题意很简单，求n内质数的个数，注意n最大也就1e6，既可用埃氏筛也可以用欧拉筛模板，水过！
AC代码：

 #include<bits/stdc++.h>

 using namespace std;

 const int maxn=1e6+;

 int n,cnt=,prime[maxn];bool isp[maxn];

 void euler_sieve(){

     memset(isp,true,sizeof(isp));

     isp[]=isp[]=false;

     for(int i=;i<maxn;++i){

         if(isp[i])prime[cnt++]=i;

         for(int j=;j<cnt&&i*prime[j]<maxn;++j){

             isp[i*prime[j]]=false;

             if(i%prime[j]==)break;

         }

     }

 }

 int main(){

     euler_sieve();

     while(~scanf("%d",&n)){

         int num=;

         for(int i=;prime[i]<=n&&i<cnt;++i)num++;

         printf("%d\n",num);

     }

     return ;

 }

巴特西