C++ Primer 7.5

7.5.cpp

#include <iostream>

long factorial(int n);

int main()
{
  using namespace std;

  int n;
  cout << "Please input a number(n): ";
  cin >> n;
  cout << "The factorial of n is: " << factorial(n) << endl;

  return 0;
}

long factorial(int n)
{
  if (0 > n) { exit(-1); /* error! */ }
  if (0 == n) { return 1; };
  return n * factorial(n - 1);
}