n阶乘

#include<iostream>
#include<limits>
using std::cin;
using std::cout;
using std::endl;

template<class T>
bool input(T& a)
{
 int i = 1;
 while(true)
 {
  cin >> a ;
  if(cin && a >= 0)
   return true;
  else
  {
   if(i == 3) return false;
   if(!cin)
   {
    cin.clear();
    cin.ignore((std::numeric_limits <int> ::max)(),   '/n');
   }
    cout << "the input is not ge than 0"
         << "do you want to try again? Y / N "
      << endl;
      char ch;
   cin >> ch;
   if(ch != 'Y' && ch != 'y' ) return false;
   ++i;
  }
 }
}

 

int fn(int n)
{
 if(n <= 1)
  return 1;
 else
  return n*fn(n-1);
}

int main()
{
 cout << "please input a number" << endl;
 int n;
 input(n) ;
 cout << "n! =/t" << fn(n) << endl;
 return 0;
}