Question

问题在于使用递归和合并排序进行排序。

我没有看到任何错误，但cmd弹出空指针异常。

import java.util.*;

public class listMethodRunner
{
    public static void main(String [] args)
    {
        ArrayList<Integer> list1 = new ArrayList<Integer>();
        list1.add(2);
        list1.add(3);
        list1.add(5);
        list1.add(9);
        list1.add(22);
        list1.add(38);
        list1.add(56);

        ArrayList<Integer> list2 = new ArrayList<Integer> ();
        list2.add(4);
        list2.add(7);
        list2.add(8);
        list2.add(23);
        list2.add(37);

        ArrayList<Integer> mergeList = ListMethods.merge(list1,list2);
        if (mergeList.size() == 0)
        {
            System.out.println("This list is empty");
        }
        else
        {
            for(Integer m: mergeList)
            {
                System.out.println(m);
            }
        }
    } // main
}  // class listMethodRunner


import java.util.*;
public class ListMethods
{
    public static ArrayList<Integer> merge(ArrayList<Integer> list1,ArrayList<Integer> list2)
    {
        ArrayList<Integer> merge = new ArrayList<Integer>();

        if (list1.size() == 0 || list2.size() == 0)
        {
            merge = new ArrayList<Integer>();
        }
        else if (list1.get(0) <= list2.get(0))
        {
            list1.remove(0);
            merge = merge(list1,list2);
            Integer temp = list1.remove(0);
            merge.add(temp);
        }
        else if (list1.get(0) >= list2.get(0))
        {
            list2.remove(0);
            Integer temp = list2.remove(0);
            merge = merge(list1,list2);
            merge.add(temp);
        }
        return merge;
    }  // merge
}  // class ListMethods

Answer 1

merge方法似乎未正确实施。以下版本基于API docs，但未经测试：

public static ArrayList<Integer> merge(ArrayList<Integer> list1,ArrayList<Integer> list2)
{
    if (list1.size() == 0) {
        merge = list2; // ... an empty first list means to return the second one 
    else if (list2.size() == 0) {
        merge = list1; // ... an empty second list means to return the first one 
    }
    else {
        ArrayList<Integer>  merge = new ArrayList<Integer>();
        Integer             temp;

        if (list1.get(0) <= list2.get(0))
        {
            temp = list1.remove(0);
        }
        else if (list1.get(0) >= list2.get(0))
        {
            temp = list2.remove(0);
        }

        merge.add(0, temp);                  // the smallest element of them all comes first ...
        merge.addAll ( merge(list1,list2) ); // ... followed by the sorted merge of both lists without the smallest element just added 
    }

    return merge;
}  // merge

如何使用递归方法进行合并排序？

1 个答案: