Factorial

A factorial is :

n! = 1 * 2 * 3 * ... * n
0! = 1
1! = 1

fact(n) = 1 * 2 * 3 * ... * (n - 1) * n
fact(n) = if n > 0 fact(n - 1) * n
          if n = 0 1

int fact(int n) {
  if (n == 0)
    return(1);
  else
    return fact(n - 1) * n;
}

Writing a recursive function is indeed easy when you have figured out the equation, you can directly write a formula into a function.

If you input a negative input, it will go into infinite recursion and get into a stack overflow. To prevent that, you can introduce a stop condition if the number is negative.