22行代码AC_Prime Number Aizu - 0009（素数筛）(解题报告)

励志用更少的代码做更高效的表达

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

#include<bits/stdc++.h> 
using namespace std;
const int M=1e6+10;
int n, 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("%dn",dp[n]);
return 0; }

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