Bubble sort adaptive (java)

问题

I'm writing up the Bubble Sort algorithm with worst case runtime of O(n^2) and best case of O(n) so that it is adaptive. My idea was to add some sort of boolean flag variable to control the while loop so that the algorithm would stop early if it was sorted early. However, it keeps failing my JUnit testing. I think it's the way I'm implementing the boolean variable but I'm not sure where else to put it. Any contributions would be greatly appreciated.

    public static<T> void bubbleSort(T[] arr, Comparator<T> comparator) {
        if (arr == null || comparator == null) {
            throw new IllegalArgumentException("Comparator nor array can be null.");
        }
        boolean swapped = true;

        while (swapped) {
            swapped = false;
            for (int i = 0; i < arr.length - 1; i++) {
                for (int j = 0; j < arr.length - i - 1, j++) {
                    if (comparator.compare(arr[j], arr[j + 1]) > 0) {
                        T obj = arr[j];
                        arr[j] = arr[j + 1]
                        arr[j + 1] = obj;
                        swapped = true;
                    }
                }
           }
       }
  }

EDIT: here are the JUNITS I am using.

public class SortingTests {
    private TeachingAssistant[] tas;
    private TeachingAssistant[] tasByName;
    private ComparatorPlus<TeachingAssistant> comp;
    private static final int TIMEOUT = 200;

@Before
public void setUp() {
        tas = new TeachingAssistant[10];
        tas[0] = new TeachingAssistant("Adrianna");
        tas[1] = new TeachingAssistant("Chad");
        tas[2] = new TeachingAssistant("Jackie");
        tas[3] = new TeachingAssistant("Miguel");
        tas[4] = new TeachingAssistant("Ashley");
        tas[5] = new TeachingAssistant("Scott");
        tas[6] = new TeachingAssistant("Tim");
        tas[7] = new TeachingAssistant("Joey");
        tas[8] = new TeachingAssistant("Raymond");
        tas[9] = new TeachingAssistant("Bartosz");
        tasByName = new TeachingAssistant[10];
        tasByName[0] = tas[0];
        tasByName[1] = tas[4];
        tasByName[2] = tas[9];
        tasByName[3] = tas[1];
        tasByName[4] = tas[2];
        tasByName[5] = tas[7];
        tasByName[6] = tas[3];
        tasByName[7] = tas[8];
        tasByName[8] = tas[5];
        tasByName[9] = tas[6];

        comp = TeachingAssistant.getNameComparator();
    }

    @Test(timeout = TIMEOUT)
    public void testBubbleSort() {
        Sorting.bubbleSort(tas, comp);
        assertArrayEquals(tasByName, tas);
        assertTrue("Number of comparisons: " + comp.getCount(),
                comp.getCount() <= 44 && comp.getCount() != 0);
}

回答1:

How about something like this:

public static<T> void bubbleSort(T[] arr, Comparator<T> comparator) {

    if (arr == null || comparator == null) {
        throw new IllegalArgumentException("Comparator nor array can be null.");
    boolean swapped = true;

    while (swapped) {
        swapped = false;
        for (int i = 0; i < arr.length - 1; i++) {
            if (comparator.compare(arr[i], arr[i + 1]) > 0) {
                T obj = arr[i];
                arr[i] = arr[i + 1]
                arr[i + 1] = obj;
                swapped = true;
            }
       }
   }


 }

回答2:

If I understood you correctly you want to achieve O(n) in case the array is already sorted (or half sorted). If you want to improve your time complexity you need to do something like this:

for (int i = 0; i < arr.length - 1; i++) {
     boolean swapped = false;
     for (int j = 0; j < arr.length - i - 1, j++) {
          if (comparator.compare(arr[j], arr[j + 1]) > 0) {
              T obj = arr[i];
              arr[i] = arr[i + 1]
              arr[i + 1] = obj;
              swapped = True;
          }
     }
     if (swapped == false)
         break; // no inner swap done so can exit the upper loop
}

来源：https://stackoverflow.com/questions/51614342/bubble-sort-adaptive-java