int factorial(int n)

#include <stdio.h>

int factorial(int);

int main(void)
{
printf("5! = %d\n", factorial(5));

return 0;
}

int factorial(int n)
{
 int result = 1;

for (int i = 2; i <= n; ++i) {
result = result * i;
}

return result;
}

---

int factorial(int n)
{
 if(n==1)
  return 1;
 return n*factorial(n-1);
}