利用递归求n！

#include<iostream>
#include<iomanip>
using namespace std;

double fac(int n) {
double y;
if (n == 0 || n == 1)
y = 1;
else
y = n * fac(n - 1);
return y;
}

int main() {
int n;
double result;
cout << "请输入不小于0的整数:";
cin >> n;
if (n >= 0) {
result = fac(n);
cout << n << "!=" << result << endl;
}
else
cout << n << "<0,请重新运行程序\n";
return 0;
}