Question

我是stackoverflow的新手，我需要一些帮助来编写一个程序来合并一个可比较的arraylist。我已经使用了几个小时的代码无济于事。程序需要正常工作，因为我是为计算机科学课做的，而下一个任务要求我们测试不同种类的效率。这是代码：

合并方法：

public static void merge(ArrayList <Comparable> a, int first, int mid, int last){
    ArrayList <Comparable> b = new ArrayList();
    for(int k = first; k <= last; k++){
        b.add(a.get(k));
    }

    System.out.println("b now contains " + b);
    int middle =b.size() /2;
    for(int i = first; i <= last; i++){
        //System.out.println("mid: " + b.size() /2);
        //System.out.println("b: " + b);
        //System.out.println("a: " + a);
        //System.out.println("i: " + i);
        if(middle == b.size()){
            a.set(i, b.remove(0));
            middle--;
        }else if(middle == 0){
            a.set(0, b.remove(0));
        }else if(b.get(0).compareTo(b.get(middle)) < 0){
            System.out.println("moving " + b.get(0) + " from b[0] to a[" + 
                i + "] because " + b.get(0) + " is less than " + b.get(middle));
            a.set(i, b.remove(0));
            middle--;
            System.out.println("b now contains " + b);
        }else{
            System.out.println("moving " + b.get(middle) + " from b[" + 
                b.size() /2 + "] to a[" + i + "] because " + b.get(0) + 
                    " is greater than " + b.get(middle));
            a.set(i, b.remove(middle));
            //middle--;
            System.out.println("b now contains " + b);
        }
    }

    System.out.println();
    System.out.println("Merge");
    System.out.println(a);
    System.out.println();
}

Mergesort方法：

public static void mergeSort(ArrayList <Comparable> a, int first, int last){
    if(first < last){
        int mid = first + (last - first) /2;
        System.out.println("mergeSorting " + a.subList(first, last + 1));
        mergeSort(a, first, mid);
        System.out.println("mergeSorting " + a.subList(first, mid + 1));
        mergeSort(a, mid + 1, last);
        System.out.println("merging " + a.subList(first, mid + 1) + 
            " and " + a.subList(mid + 1, last + 1));
        merge(a, first, mid, last);
    }
    System.out.println();
    System.out.println("base case");
    System.out.println();
}

我认为merge方法存在问题，但我不确定。

我的代码似乎错误地对列表进行排序，即：

Input:
[7,1,9,9,5,4,8,9,10,4] 

Output:
[4,8,9,4,10,10,5,9,9,7]

Answer 1

mergeSort算法没问题。我刚刚定义了ArrayList的类型以避免警告

   public static void mergeSort(ArrayList <Comparable> a, int first, int last){
        if(first < last){
            int mid = first + (last - first) /2;
        //    System.out.println("mergeSorting " + a.subList(first, mid ));
            System.out.println("First"+first);
            System.out.println("Mid"+mid);
            System.out.println("Last"+last);
            System.out.println("MergeSortCall");
            System.out.println("firstVector " + a.subList(first, mid+1));
            System.out.println("secondVector " + a.subList(mid + 1,last+1));                
            mergeSort(a, first, mid);
            mergeSort(a, mid + 1, last);
            merge(a, first, mid, last);
            System.out.println("mergeVector " + a.subList(first,last+1));
        }

    }

第二部分真的很难确切地知道出了什么问题所以我用基本上你的符号更简单地替换了它。请注意，第二部分只是一个“合并”算法。它应该是非常简单的。无论如何，使用这个你可以比较你的结果（对不起，很难理解第二部分的逻辑，这是你的功课，不是吗？）。我正在使用两个向量来提高可读性，但只有一个像你所做的那样是必要的。

public static void merge(ArrayList <Comparable> a, int first, int mid, int last){
            ArrayList <Comparable> vectorFirst = new ArrayList<Comparable>();
            ArrayList <Comparable> vectorSecond = new ArrayList<Comparable>();

            for(int k=first;k<=mid;k++){
                vectorFirst.add(a.get(k));
            }
            for(int k=mid+1;k<=last;k++){
                vectorSecond.add(a.get(k));
            }

            int i=first;
            int j=mid+1;
            int k=first-1;
            while((i<=mid)&(j<=last)){
                if(vectorFirst.get(i-first).compareTo(vectorSecond.get(j-mid-1))==-1){
                    k=k+1;
                    a.set(k,vectorFirst.get(i-first));
                    i=i+1;
                }
                else{
                     k=k+1;
                     a.set(k,vectorSecond.get(j-mid-1));
                     j=j+1;
                }
            }
            if(i==mid+1){
                while(j<=last){
                    k=k+1;
                    a.set(k,vectorSecond.get(j-mid-1));
                    j=j+1;
                }
            }
            else{
                while(i<=mid){
                    k=k+1;
                    a.set(k,vectorFirst.get(i-first));
                    i=i+1;
                }
            }


        }

java中的合并排序问题

1 个答案: