Given an array of integers (without any duplicates), shuffle the array such that all permutations are equally likely to be generated.

Solution: 从大到小，每次在[0, i]中随机产生一个index，再swap(array, index, i)

产生[0, x) int的方法：Math.random().nextInt(x)或(int)(Math.random() * x)

  public void shuffle(int[] array) {
    if (array.length < 2) {
      return;
    }
    Random rand = new Random();
    for (int i = array.length - 1; i >= 1; i--) {
      int index = rand.nextInt(i + 1);
      int temp = array[i];
      array[i] = array[index];
      array[index] = temp;
    }
  }