勵志用更少的代碼做更高效的表達

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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

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#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("%d\n",dp[n]); return 0; }

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