Quicksort（Random Pivot）代码没有正确排序Java

Question

代码是一个带有随机数据的快速排序算法。 它没有完全对随机数组进行排序； 我无法弄清楚为什么。 错误似乎与递归调用和交换的边界有关。

    public static <E extends Comparable<E>> void quickerSort(ArrayList<E>         input, int lower, int higher){
            int first = lower;
            int last = higher;
            if (lower >= higher)
            return;
        //generates the random pivot
            int pivot = rand.nextInt(higher-lower+1) + lower;

        //moves the pivot to the beginning of the list

            E temp = input.get(first);
            input.set(first, input.get(pivot));
            input.set(pivot, temp);
            pivot = first;
            first++;

      //if the values on the left side of the array are less than the 
      //value of the pivot, first is incremented (first part of loop) until
      //a greater value is reached  

      //if the values on the right side of the array are greater than the
      //value of the pivot, last is incremented until a lesser value is reached

            while (first < last){
                  while (input.get(first).compareTo(input.get(pivot)) <= 0 && first < last){
                         first++;
            }
                  while(input.get(last).compareTo(input.get(pivot)) > 0 && last >= first){
                         last--;
            }

       //switches the two values reached through the while loops
            if (first < last){
                temp = input.get(first);
                input.set(first, input.get(last));
                input.set(last, temp);
            }
        }

     //moves the pivot to where first is
        temp = input.get(first-1);
        input.set(first-1, input.get(pivot));
        input.set(pivot, temp);

     //calls the method recursively
        if (lower < first-1){
            quickerSort(input,lower,first-1);
        }
        if (first < higher){
            quickerSort(input,first,higher);

        }
    }   

//实现方法

public static void testQuickSort(){

      //creates a random list of integers to be sorted and prints it
        ArrayList<Integer> list = new ArrayList<Integer>();
        Random rand = new Random();
        for (int i = 0; i <= 19; i++){
            list.add(rand.nextInt(100));
            System.out.print(list.get(i)+" ");
    } 
      //calls quicksort and then prints the sorted array
        System.out.println();
        Assignment2.quickSort(list);
        for (int i = 0; i <= 19; i++){
            System.out.print(list.get(i)+" ");
    }
    }

Answer 1

你的代码不能用于简单输入{6,0}，因为你的first和last都将指向第二个元素，因此，6和0永远不会被交换。

我修改了你的代码并且它有效。 我使用的变量名在这里更有意义，但逻辑是一样的。

public static <E extends Comparable<E>> void quickSort(ArrayList<E> input, int low, int high) {
    if (low >= high)
        return;

    // generates the random pivot
    int pivotIndex = new Random().nextInt(high - low + 1) + low;

    E pivot = input.get(pivotIndex);

    // moves the pivot to the beginning of the list
    Collections.swap(input, pivotIndex, low);

    int i = low;
    int j = high;

    // if the values on the left side of the array are less than the
    // value of the pivot, first is incremented (first part of loop) until
    // a greater value is reached

    // if the values on the right side of the array are greater than the
    // value of the pivot, last is incremented until a lesser value is
    // reached

    while (i <= j) {
        while (input.get(i).compareTo(pivot) <= 0 && i < j) {
            i++;
        }
        while (input.get(j).compareTo(pivot) > 0 && j >= i) {
            j--;
        }

        // break here when two pointers meet since the pivot position has been found
        if (i >= j)
            break;

        // switches the two values reached through the while loops
        Collections.swap(input, i, j);
    }

    // moves the pivot to where j is
    Collections.swap(input, low, j);

    // calls the method recursively
    quickSort(input, low, j - 1);

    quickSort(input, j + 1, high);

}