有人能解释一下这种快速排序方法吗？

Question

你们能解释一下这种快速排序方法吗？我正在尝试将这段代码实现到我的程序中，但是作者没有解释它做了什么，或者它是如何实现的。也请注意，我在高中，所以请尽量保持它可以理解。

我所知道的是它加快了二维数组的速度。我还知道它使用递归来执行它的快速排序。不幸的是，就是这样。任何帮助都将不胜感激。

public double[][] quicksort(double[][] array, int key, int down, int top) {
    double[][] a = new double[array.length][2];
    System.arraycopy(array,   0, a, 0, a.length);

    int i = down;
    int j = top;
    double x = a[(down + top) / 2][key];

    do {
      while (a[i][key] < x) {
        i++;
      }
      while (a[j][key] > x) {
        j--;
      }
      if (i <= j) {
        double[] temp = new double[a[i].length];

        for (int y = 0; y < a[i].length; y++) {
          temp[y] = a[i][y];
          a[i][y] = a[j][y];
          a[j][y] = temp[y];
        }
        i++;
        j--;
      }
    } while (i <= j);

    if (down < j) {
      a = quicksort(a, key, down, j);
    }

    if (i < top) {
      a = quicksort(a, key, i, top);
    }

    return a;
  }
}

Answer 1

有几件事要知道：

• array是键值对的数组，它是按键排序的.
• 此快速排序返回原始数组的副本，而不是更改现有的数组。

见评论：

public double[][] quicksort(double[][] array, int key, int down, int top) {
    //create copy of array (the author wanted to return a new one)
    double[][] a = new double[array.length][2];
    System.arraycopy(array,   0, a, 0, a.length);

    int i = down; //lower limit
    int j = top;  //upper limit
    double x = a[(down + top) / 2][key]; //the pivot

    do {
      while (a[i][key] < x) { //skip over smaller elements in beginning
        i++;
      }
      while (a[j][key] > x) { //skip over larger elements in end
        j--;
      }

      //now do some partitioning
      if (i <= j) {
        //create temporary array, for swapping elements
        double[] temp = new double[a[i].length];

        for (int y = 0; y < a[i].length; y++) {
          temp[y] = a[i][y];
          a[i][y] = a[j][y];
          a[j][y] = temp[y];
        }
        i++;
        j--;
      }
    } while (i <= j);

    //if there is a non-empty lower partition, sort that
    if (down < j) {
      a = quicksort(a, key, down, j);
    }

    //if there is a non-empty upper partition, sort that
    if (i < top) {
      a = quicksort(a, key, i, top);
    }

    //return the result
    return a;
  }
}