题意：

给一个数n，问1~n内有多少个素数

题目：

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

分析：

打个素数筛就行

AC模板：

#include<stdio.h> #include<string.h> #include<map> #include<algorithm> using namespace std; const int M=1e6+10; int n; int dp[M],book[M]; map<int,int>mp; void init() { for(int i=2; i<M; i++) { if(!book[i]) { mp[i]=1; for(int j=i; j<M; j+=i) book[j]=1; } } for(int i=1; i<M; i++) { dp[i]=dp[i-1]; if(mp[i]) dp[i]++; } } int main() { init(); while(~scanf("%d",&n)) { printf("%d\n",dp[n]); } return 0; }

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