int GCD ( int a , int b) //遞迴版
{
    if ( b != 0 )  return GCD( b, a%b );
    return a;
}

int GCD ( int a , int b) //非遞迴版
{
    int c;
    while (b)
    {
        c = a;
        a = b;
        b = c % a;
    }
    return a;
}

另一種方法：

若x,y均為偶數，gcd(x,y) = 2 * gcd(x/2,y/2);

若只x均為偶數，gcd(x,y) = gcd(x/2,y);

若只y均為偶數，gcd(x,y) =  gcd(x,y/2);

若x,y均為奇數，gcd(x,y) = gcd(y, x- y);(兩個奇數相減，必得偶數)