題意：

給一個數n，問1~n內有多少個素數

題目：

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：

打個素數篩就行

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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