！壞人必需做好事！

改進兩點，
一．用根號縮小判斷範圍
二．用+2取代+1減少要判斷的數
/*
  Name: 質因數判斷加強版，感謝某網友提供想法
  Date: 19/06/08 09:59
*/
#include <iostream>
#include <cmath>
int main()
{
          using namespace std;
          double Num2;
          unsigned long Num,bh,x;
          char a;
          bool temp;
          again:
          cout << "請輸入一個正整數：__________\b\b\b\b\b\b\b\b\b\b";
          cin >> Num;
          Num2 = sqrt(Num);
          temp = true;
          if (Num == 2)
               goto exit;
          if (Num % 2 == 0 or Num <2){
               temp = false;
               goto exit;
          }
          for (unsigned long i = 3;i <= Num2;i = i + 2){
                  if (Num % i == 0){
                  temp = false;
                  goto exit;
                  }
          }
          exit:
          if (temp == false)
          cout << "\n不是質數。\n";
          else
          cout << "\n是質數。\n";
          cout << "是否繼續？(y/n)";
          cin.get();
          a = getchar();
          system("cls");
          if (a == 'y')
          goto again;
          else
          //system("pause");
          return 0;
}