水池抽樣， reservoir sample

static int[] randomPick(int[] array, int k) { 
    int[] res = new int[k];
    Random r = new Random();
    for (int i = 0; i < k; i++) {
      res[i] = array[i];
    }
    for (int i = k; i < array.length; i++) {
      int index = r.nextInt(i);
      if (index < k) {
        res[index] = array[i];
      }
    }
    return res;
  }