不正确的合并排序答案

【问题标题】：Incorrect Merge Sorting不正确的合并排序
【发布时间】：2017-11-04 05:20:06
【问题描述】：

我正在开发一个按标题字母顺序对电影进行排序的合并排序程序。但是，它不会对列表进行排序以完成。我研究了归并排序，并查看了本网站和其他地方的各种其他示例，但找不到我的错误。

public static Movie4[] myMovies;

/**
 * printMovies traverses an array of movies and prints each respective title
 */
public static void printMovies(Movie4[] movies)
{
    for (int i = 0; i < movies.length; i ++)
    {
        System.out.println(movies[i]);
    }
}

/**
 * sortTitles sorts an array of movies alphabetically by title
 */
public static void sortTitles(Movie4[] movies, int low, int high)
{
    if (low == high)
    {
        return;
    }
    else
    {
        Movie4[] firstHalf = new Movie4[movies.length  / 2];
        Movie4[] secondHalf = new Movie4[movies.length - movies.length / 2];
        int midpoint = (low + high) / 2;
        for (int i = 0; i < firstHalf.length; i ++)
        {
            firstHalf[i] = movies[i];
        }
        for (int i = 0; i < secondHalf.length; i ++)
        {
            secondHalf[i] = movies[i + movies.length / 2];
        }
        sortTitles(firstHalf, low, midpoint);
        sortTitles(secondHalf, midpoint + 1, high);
        mergeTitles(movies, firstHalf, secondHalf);
    }
}

/**
 * mergeTitles works in conjunction with sortTitles to merge sort an array of Movie4 objects 
 */
public static void mergeTitles(Movie4[] movies, Movie4[] first, Movie4[] second)
{
    int i = 0;
    int i2 = 0;
    for (int index = 0; index < movies.length; index ++)
    {
        if (i2 >= second.length || (i < first.length && 
                                        first[i].getTitle().compareTo(second[i2].getTitle()) < 0))
        {
            movies[index] = first[i];
            i ++;
        }
        else
        {
            movies[index] = second[i2];
            i2 ++;
        }
    }
}

public static void main(String[] args)
{
    myMovies = new Movie4[10];
        myMovies[0] = new Movie4("The Muppets Take Manhattan", 2001, "Columbia Tristar");
        myMovies[1] = new Movie4("Mulan Special Edition", 2004, "Disney");
        myMovies[2] = new Movie4("Shrek 2", 2004, "Dreamworks");
        myMovies[3] = new Movie4("The Incredibles", 2004, "Pixar");
        myMovies[4] = new Movie4("Nanny McPhee", 2006, "Universal");
        myMovies[5] = new Movie4("The Curse of the Were-Rabbit", 2006, "Aardman");
        myMovies[6] = new Movie4("Ice Age", 2002, "20th Century Fox");
        myMovies[7] = new Movie4("Lilo and Stich", 2002, "Disney");
        myMovies[8] = new Movie4("Robots", 2005, "20th Century Fox");
        myMovies[9] = new Movie4("Monsters Inc.", 2001, "Pixar");

    System.out.println("Original List:");
    System.out.println();
    printMovies(myMovies);
    System.out.println();
    System.out.println("List Sorted by title, ascending:");
    System.out.println();
    sortTitles(myMovies, 0, myMovies.length - 1);
    printMovies(myMovies);
}

请假设getTitle() 方法在别处定义，并且只返回每个对象的第一个String，即标题。这是错误的输出；

List Sorted by title, ascending:

Ice Age, 2002, 20th Century Fox.
Lilo and Stich, 2002, Disney.
Mulan Special Edition, 2004, Disney.
Robots, 2005, 20th Century Fox.
Monsters Inc., 2001, Pixar.
Shrek 2, 2004, Dreamworks.
The Curse of the Were-Rabbit, 2006, Aardman.
The Incredibles, 2004, Pixar.
Nanny McPhee, 2006, Universal.
The Muppets Take Manhattan, 2001, Columbia Tristar.

我认为错误可能与我的条件语句有关，因为如果我将 compareTo 值设置为 > 0，它会正确排序，只是顺序错误。

【问题讨论】：

我建议学习使用调试器。
您的代码看起来像是自顶向下 merge sort 的实现。你考虑过自下而上的方法吗？对我来说感觉更直观。我还将根据[Comparable] 实现该算法，并为Movie4 提供适当的比较方法（单线）。

标签： java string sorting merge

【解决方案1】：

您不应该分配新数组或复制元素。 sortTitles() 中的所有内容都应使用 low 和 high 进行管理。同样，您不应将三个数组传递给mergeTitles()；相反，传递单个数组和值low、midpoint 和high，让它知道要合并数组的哪些部分。

您当前的代码似乎有一个栅栏问题：您使用(low + high) / 2 计算midpoint，但使用movies.length / 2 作为第一个数组的长度。由于low 以0 开头，high 以myMovies.length - 1 开头，因此当myMovies.length 为偶数时，这些长度会有所不同。您可能还有其他索引问题，但我没有深入研究，因为整个方法存在缺陷，正如我在第一段中描述的那样。

【讨论】：