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

10
3
11

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+5;
int n,cnt=0,prime[maxn];bool isp[maxn];
void euler_sieve(){
    memset(isp,true,sizeof(isp));
    isp[0]=isp[1]=false;
    for(int i=2;i<maxn;++i){
        if(isp[i])prime[cnt++]=i;
        for(int j=0;j<cnt&&i*prime[j]<maxn;++j){
            isp[i*prime[j]]=false;
            if(i%prime[j]==0)break;
        }
    }
}
int main(){
    euler_sieve();
    while(~scanf("%d",&n)){
        int num=0;
        for(int i=0;prime[i]<=n&&i<cnt;++i)num++;
        printf("%d\n",num);
    }
    return 0;
}