Recursions

using System;

namespace Recursions
{
    internal class Program
    {
        static int Factorial(int n)
        {
            if (n == 0)
            {
                return 1;
            }

            return n * Factorial(n - 1);
        }

        static int Fib(int n)
        {
            if (n == 0 || n == 1)
            {
                return 1;
            }
            return Fib(n - 2) + Fib(n - 1);
        }

        static void NToOne(int n)
        {
            if (n == 0)
            {
                return;
            }
            Console.WriteLine(n);
            NToOne(n - 1);
        }

        static void OneToN(int n)
        {
            if (n == 0)
            {
                return;
            }
            OneToN(n - 1);
            Console.WriteLine(n);
        }

        // O(n)
        static double Power(double a, int n)
        {
            if (n == 0)
            {
                return 1;
            }
            if (n < 0)
            {
                return 1 / Power(a, -n);
            }
            return a * Power(a, n - 1);
        }

        // O(log(n))
        static double FastPower(double a, int n)
        {
            if (n == 0)
            {
                return 1;
            }
            if (n < 0)
            {
                return 1 / FastPower(a, -n);
            }
            if (n % 2 == 0)
            {
                double x = FastPower(a, n / 2);
                return x * x;
            }
            double y = FastPower(a, (n - 1) / 2);
            return a * y * y;
        }

        static int GCD(int a, int b)
        {
            if (b == 0)
            {
                return a;
            }
            return GCD(b, a % b);
        }

        static void Main(string[] args)
        {
            Console.WriteLine(GCD(250, 100));
        }
    }
}