java - MergeSort 和 insertionSort 组合算法一起运行比单独运行慢但应该运行得更快

Question

我有一个合并排序需要在特定数字切换到插入排序，这是我的阈值。但我的阈值只能是 1、2 或 3，因为在任何其他情况下，我的合并排序都会变得非常慢。我似乎无法让代码协同工作。

这是我的代码:

public class InsertionSort {

    // I haven't found the right Threshold yet, but it should work with any number between 1-100.

public final static int M = 16;

    private void Merge(int arr[], int left, int mid, int right) {
        int size1 = mid - left + 1;
        int size2 = right - mid;

        int LeftArray[] = new int[size1];
        int RightArray[] = new int[size2];

        for (int i = 0; i < size1; ++i) {
            LeftArray[i] = arr[left + i];

        }
        for (int j = 0; j < size2; ++j) {
            RightArray[j] = arr[mid + 1 + j];

        }

        int i = 0;
        int j = 0;
        int k = left;

        while (i < size1 && j < size2) { 
            if (LeftArray[i] <= RightArray[j]) {
                arr[k] = LeftArray[i];
                i++;
            } else {
                arr[k] = RightArray[j];
                j++;
            }
            k++;
        }


        while (i < size1) {
            arr[k] = LeftArray[i];
            i++;
            k++;
        }

        while (j < size2) {
            arr[k] = RightArray[j];
            j++;
            k++;
        }

    }

    public void MergeSort(int arr[], int left, int right, int M) {

        if ( left < right ) {

            int mid = (left + right) / 2;
            MergeSort(arr, left, mid, M);
            MergeSort(arr, (mid+1), right, M);
            Merge(arr, left, mid, right);
    } else if ((right - left + 1) <= M)  {
        insertion_sort(arr, left, right);
    }}


    static void printArray(int arr[]) {
        int n = arr.length;
        for (int i = 0; i < n; ++i)
            System.out.print(arr[i] + " ");
        System.out.println();
    }

    static int[] readIntfile(String filename) throws Exception {
        // Read file into a byte array, and then combine every group of four bytes to an
        // int. (Not
        // the standard way, but it works!)
        byte[] bytes = Files.readAllBytes(Paths.get(filename));
        int[] ints = new int[bytes.length / 4];
        for (int i = 0; i < ints.length; i++) {
            for (int j = 0; j < 4; j++) {
                ints[i] += (bytes[i * 4 + j] & 255) << (3 - j) * 8;
            }
        }
        return ints;

    }

    public static void insertion_sort(int a[], int left, int right) {
        int j;
        for (int i = left; i <= right; i++) {
            int tmp = a[i];
            for (j = i; j > 0 && tmp < a[j - 1]; j--) {
                a[j] = a[j - 1];
            }
            a[j] = tmp;
        }
    }


    public static void main(String args[]) throws Exception {
       // I have texfile named this with 1000000 numbers
         int arr[] = readIntfile("smallints");

         // you can also try with this array for example
    // int arr[] = {3, 6, 4, 8, 500, 1, 5, 10, 7, 9, 0, 2, 100, 300, 1000, 20, 13, 17, 55, 93};
        InsertionSort insert = new InsertionSort();
        long before = System.currentTimeMillis();
        insert.MergeSort(arr, 0, arr.length-1, M);
        long after = System.currentTimeMillis();
        printArray(arr);
        System.out.println("\n" + "Done " + ((after - before) / 1000.0 + " sek"));
    }
}

Answer 1

public void MergeSort(int arr[], int left, int right, int M) {

    if ( left < right ) {

        int mid = (left + right) / 2;
        MergeSort(arr, left, mid, M);
        MergeSort(arr, (mid+1), right, M);
        Merge(arr, left, mid, right);
} else if ((right - left + 1) <= M)  {
    insertion_sort(arr, left, right);
}}

你这里有一个严重的问题。您有一个类全局变量 M，它是您的插入排序阈值，您有一个参数 M，它是您要排序的子数组的长度。该参数将覆盖全局类。基本上，您的 public final static int M = 16; 从未在 MergeSort 方法中出现。

此外，如果 left 不小于 right，则没有可排序的。我想你想要这样的东西:

public final static int insertionThreshold = 16;

public void MergeSort(int arr[], int left, int right, int M) {
    if ( left < right ) {
        if (left - right <= insertionThreshold) {
            insertion_sort(arr, left, right);
        else {
            int mid = (left + right) / 2;
            MergeSort(arr, left, mid, M);
            MergeSort(arr, (mid+1), right, M);
            Merge(arr, left, mid, right);
        }
    }
}