Question

我想比较排序算法Merge Sort和Selection Sort的操作次数，但是我有一些问题要弄清楚要计算哪些操作以及哪些操作没有...

这是我的实现。我想我正在以正确的方式计算Selection Sort的操作，但我不知道Merge Sort：

选择排序：

public class SelectionSort {
private int exchanges, comparisons;

public void selectionSort(int[] x) {    
    for (int i=0; i<x.length-1; i++) {
        for (int j=i+1; j<x.length; j++) {
            if (x[i] > x[j]) 
            {
                //... Exchange elements
                int temp = x[i];
                x[i] = x[j];
                x[j] = temp;
                exchanges++;
            }
            comparisons++;
        }
    }
    System.out.println("SelSort_Exchanges: "+exchanges);
    System.out.println("SelSort_Comparisons: "+comparisons);
    System.out.println("SelSort_Operations: "+(exchanges+comparisons));
}
}

如果我输入10 int的数组，我得到：

SelSort_Comparisons: 45
SelSort_Exchanges: 27
SelSort_Operations: 72

似乎对我而言，但现在对于Merge Sort：

public class Mergesort {
private int[] numbers;
private int[] helper;

private int number;
private int comparisons, exchanges;

public void sort(int[] values) {
    this.numbers = values;
    number = values.length;
    this.helper = new int[number];
    mergesort(0, number - 1);
    System.out.println("MerSort_Comparisons "+comparisons);
    System.out.println("MerSort_Exchanges "+exchanges);
    System.out.println("MerSort_Operations "+(comparisons+exchanges));
    System.out.println();
}

private void mergesort(int low, int high) {
    // Check if low is smaller then high, if not then the array is sorted
    if (low < high) 
    {
        // Get the index of the element which is in the middle
        int middle = (low + high) / 2;
        // Sort the left side of the array
        mergesort(low, middle);
        // Sort the right side of the array
        mergesort(middle + 1, high);
        // Combine them both
        merge(low, middle, high);
    }
}

private void merge(int low, int middle, int high) {

    // Copy both parts into the helper array
    for (int i = low; i <= high; i++) {
        helper[i] = numbers[i];
        exchanges++;
    }
    int i = low;
    int j = middle + 1;
    int k = low;
    // Copy the smallest values from either the left or the right side back
    // to the original array
    while (i <= middle && j <= high) {
        if (helper[i] <= helper[j]) {
            numbers[k] = helper[i];
            i++;
            exchanges++;
        } else {
            numbers[k] = helper[j];
            j++;
            exchanges++;
        }
        k++;
        comparisons++;

    }
    // Copy the rest of the left side of the array into the target array
    while (i <= middle) {
        numbers[k] = helper[i];
        exchanges++;
        k++;
        i++;
    }
}   
}

现在我得到了

MerSort_Comparisons 22
MerSort_Exchanges 61
MerSort_Operations 83

因此我不知道它是否正确。比较的结果对我来说似乎是正确的，但是如果我采用20的数组，例如它似乎不再正确。

任何人都可以帮我解决这个问题并告诉我在哪里确切地进行比较和交换增量？

提前谢谢！：）

Answer 1

最简单的方法是创建两个方法Compare和Swap，并在其中增加计数器。无论你实现什么sorting algorithm，它都应该只使用这些方法与数组交互。

此外，this page可以帮助您直观地分析不同的排序算法。

计算合并排序和选择排序的操作

1 个答案: