Prime Number（数学 + 素数筛选）

     

Prime Number

Time Limit : 1 sec, Memory Limit : 65536 KB 

Prime Number

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, 7.

Input

Input consists of several datasets. Each dataset has an integer n (n ≤ 999999) in a line.

The number of datasets ≤ 30.

Output

For each dataset, prints the number of prime numbers.

Sample Input

10
3
11

Output for the Sample Input

4
2
5

 

 

 

      题意：

      输出 N （<= 999999）以内素数的个数。

 

      思路：

      素数筛选法。

 

      AC：

#include <stdio.h>
#include <string.h>
#define MAX 1000000

int pri[MAX];

void make_pri() {
    memset(pri,0,sizeof(pri));
    for(int i = 2;i < MAX;i++) {
        if(pri[i]) continue;
        for(int j = i + i;j < MAX;j += i)   pri[j] = 1;
    }
}

int main() {
    int n;
    make_pri();

    while(~scanf("%d",&n)) {
        int sum = 0;
        for(int i = 2;i <= n;i++)
            if(!pri[i]) sum++;
        printf("%d\n",sum);
    }

    return 0;
}