扩展欧几里德算法

x_k+1 = y_k , y_k+1 = x_k - a/b*y_k ，(其中，x₀ = 1, y₀ = 0)。

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