用递归方法求 n!

 #include <iostream>

 using namespace std;
 #define LL long long

 LL fac(int n)
 {
     LL f;
     if(n ==  || n == )
         f = ;
     else f = n * fac(n - );
     return f;
 }

 int main()
 {
     LL n;
     while(cin >> n)
     {
         cout << fac(n) << endl;
     }
     return ;
 }