随机打乱一个数组

public static void shuffle(Object[] a) {
    int len = a.length;
    for (int i = 0; i < len; i++) {
        // get a random int from 0 to i
        int r = StdRandom.uniform(i + 1);
        // change the positon of i and r
        swap(a, i, r);
    }
}