Question

以下是我对合并排序的实现。

private static void mergeSort(int[] a, int low , int high,int[] res)
{
    int mid = (low + high)  /2;
    if (low  < high)
    {
        mergeSort(a,low,mid-1,res);
        mergeSort(a,mid,high-1,res);
        merge(a,low,mid,high,res);

    }
}



   private static void merge(int[] a, int low , int mid , int high,int[] res)
{

    int i = low;
    int j = mid ;

    int k =0;

    while (i < mid && j < high)
        if(a[i] < a[j])
               res[k++] = a[i++];
        else
              res[k++] = a[j++];


    while(i < mid)
        res[k++] = a[i++];

    while(j < high)
        res[k++] =a[j++];
}

当我在main方法中运行此程序时，我打印出原始数组。不确定是什么问题。当我单独测试时，合并方法有效。

public static void main(String[] args)
{
    int[] a = {45,24,53,13,54,45,63,23};
    int[] res = new int[a.length];
    mergeSort(a,0,a.length,res);
    for(int i=0 ; i < res.length ; i++)
    {
       System.out.print(res[i] +",");
    }
}

输出

 45,24,53,13,54,45,63,23,

我花了很多时间寻找问题。我无法修复它。

Answer 1

问题实际上相当复杂。

主要问题是您只是合并到res，但您再也不会使用它了。所以你最终会用每个递归级别覆盖它。

以下是在a和res之间来回合并的修补版本。它确实会破坏a的内容，所以它可能不是你想要的。

private static void mergeSort(int[] a, int low , int high,int[] res)
{
    int mid = (low + high)  /2;
    if (low + 1 < high)
    {
        //  Sort sub-parts
        mergeSort(a,low,mid,res);
        mergeSort(a,mid,high,res);

        //  Copy back to "a"
        for (int c = low; c < high; c++){
            a[c] = res[c];
        }

        //  Merge back to "res"
        merge(a,low,mid,high,res);
    }else{
        res[low] = a[low];
    }
}

private static void merge(int[] a, int low , int mid , int high,int[] res)
{

    int i = low;
    int j = mid;

    int k = low;   //  Use "low" instead of 0.

    while (i < mid && j < high)
        if(a[i] < a[j])
               res[k++] = a[i++];
        else
              res[k++] = a[j++];


    while(i < mid)
        res[k++] = a[i++];

    while(j < high)
        res[k++] =a[j++];
}

输出：

13,23,24,45,45,53,54,63,

Answer 2

答案是@Mysticial sais，除了需要在merge方法中复制数组：

private static void mergeSort(int[] a, int low , int high,int[] res)
{
    int mid = (low + high)  /2;
    if (low + 1 < high)
    {
        //  Sort sub-parts
        mergeSort(a,low,mid,res);
        mergeSort(a,mid,high,res);

        //  Merge back to "res"
        merge(a,low,mid,high,res);
    }else{
        res[low] = a[low];
    }
}

private static void merge(int[] a, int low , int mid , int high,int[] res)
{

    int i = low;
    int j = mid;

    int k = low;   //  Use "low" instead of 0.

    while (i < mid && j < high)
        if(a[i] < a[j])
               res[k++] = a[i++];
        else
              res[k++] = a[j++];


    while(i < mid)
        res[k++] = a[i++];

    while(j < high)
        res[k++] =a[j++];

    //  Copy back to "a"
        for (int c = low; c < high; c++){
            a[c] = res[c];
        }

}

无论如何，请注意这样做会覆盖原始数组......因此您可能希望将调用包装为mergeSort以避免它：

private static int[] mergeSort(int[] a){
    int[] b = new int[a.length];
    int[] tmp = new int[a.length];
    System.arraycopy(a, 0, b, 0, a.length);
    mergeSort(b, 0, b.length, tmp);
    return b;
}

public static void main(String[] args) {
    int[] a = {45, 24, 53, 13, 54, 45, 63, 23};
    int[] res = mergeSort(a);
    for (int i = 0; i < res.length; i++) {
        System.out.print(res[i] + ",");
    }
}

希望这有帮助！

为什么这种合并排序实现不起作用？

2 个答案: