PAT(B)1007. 素数对猜想 (20)

思路：用筛选法求出n内所有素数

#include <iostream>using namespace std;const int range = 100000;bool IsPrime[range + 1];void set(bool IsPrime[],int range){int i, j;for (i = 0; i <= range; ++i)IsPrime[i] = true;IsPrime[0] = IsPrime[1] = false;for (i = 2; i <= range; ++i){if (IsPrime[i]){for (j = 2 * i; j <= range; j += i)IsPrime[j] = false;}}}int main(){int n;int ct = 0;

  cin >> n;set(IsPrime, n);for (int i = 0; i < n; ++i){if (IsPrime[i] && IsPrime[i + 2])++ct;}cout << ct << endl;}