扩展 GCD板子

 扩展 GCD  
 求x, y使得gcd(a, b) = a * x + b * y;  

int extgcd(int a, int b, int & x, int & y)
{ 
    if (b == 0) { x=1; y=0; return a; } 
    int d = extgcd(b, a % b, x, y); 
    int t = x; x = y; y = t - a / b * y; 
    return d; 
}