C#:最大公約数を求める （ユークリッドの互除法） ref: https://qiita.com/gushwell/items/e9614b4ac2bea3fc6486

gcd.cs

 class Program {
     static void Main(string[] args) {
         Console.WriteLine(Gcd(0, 8));
         Console.WriteLine(Gcd(12, 8));
         Console.WriteLine(Gcd(3400, 6596));
         Console.WriteLine(Gcd(12707, 12319));
     }
     // ユークリッドの互除法 
     public static int Gcd(int a, int b) {
         if (a < b)
             // 引数を入替えて自分を呼び出す
             return Gcd(b, a);
         while (b != 0) {
             var remainder = a % b;
             a = b;
             b = remainder;
         }
         return a;
     }
 }

gcdRecursive.cs

 public static int Gcd(int a, int b) {
     return a > b ? GcdRecursive(a, b) : GcdRecursive(b, a);
 }

 private static int GcdRecursive(int a, int b) {
     return b == 0 ? a : GcdRecursive(b, a % b);
 }

gcdRecursive2.cs

public static int Gcd(int a, int b) {
    Func<int, int, int> gcd = null;
    gcd = (x, y) => y == 0 ? x : gcd(y, x % y);
    return a > b ? gcd(a, b) : gcd(b, a);
}