欧几里得/拓展欧几里得

inline int gcd(int a,int b){return (b==0)?a:gcd(b,a%b);}

inline int lcm(int a,int b){return a/gcd(a,b)*b;}

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

///递归写法
///复制自百度

int exgcd( int a,int b, int &x, int &y )
{
    int r = a % b;
    int x0, y0, x1, y1;
    x0 = 1; y0 = 0;
    x1 = 0; y1 = 1;
    x = x1, y = y1;
    while( r )
    {
        x = x0 - a / b * x1;
        y = y0 - a / b * y1;
        x0 = x1;
        y0 = y1;
        x1 = x;
        y1 = y;
        a = b;
        b = r;
        r = a % b;
    }
    return b;
}
//循环写法
//复制自简书
//不太理解exgcd，还没有验证