AIZU 0009

Prime Number

Time Limit : 1 sec, Memory Limit : 65536 KB
Japanese version is here

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

水题，素数筛选。

 #include <cstdio>
 #include <cstring>
 #include <algorithm>
 #include <iostream>

 using namespace std;

 #define maxn 999999 + 5
 typedef long long ll;

 int prime[maxn];

 void init() {
         for(int i = ; i <= maxn - ; i++) {
                  prime[i] = i % ;
         }

         prime[] = ;

         for(int i = ; i * i <= maxn; i++) {
                 if(!prime[i]) continue;
                 for(int j = i; j * i <= maxn; j++) {
                         prime[j * i] = ;
                 }
         }

         for(int i = ; i <= maxn - ; i++) {
                 prime[i] += prime[i - ];
         }
 }

 int main() {
         int n;
         init();
         while( ~scanf("%d",&n)) {
                 printf("%d\n",prime[n]);
         }

         return ;

 }