Question

有人可以告诉我下面的mergesort实现有什么问题吗？我一直在摸不着头几个小时..

void merge(int arr[], int low, int mid, int high)
{
    int i = 0;
    int j = 0;
    int k = 0;

    int buf1[1024];
    int buf1Length = mid - low + 1;

    int buf2[1024];
    int buf2Length = high - (mid + 1) + 1;

    for (i = low; i <= mid; i++)
    {
        buf1[j++] = arr[i];
    }

    for (i = mid + 1; i <= high; i++)
    {
        buf2[k++] = arr[i];
    }

    i = 0;
    j = 0;
    k = 0;

    while (j < buf1Length && k < buf2Length)
    {
        if (buf1[j] <= buf2[k])
        {
            arr[i++] = buf1[j++];
        }
        else
        {
            arr[i++] = buf2[k++];
        }
    }

    while (j < buf1Length)
        arr[i++] = buf1[j++];

    while (k < buf2Length)
        arr[i++] = buf2[k++];
}

void mergeSort(int arr[], int low, int high)
{
    int mid;

    if (low < high)
    {
        mid = (low + high) / 2;
        mergeSort(arr, low, mid);
        mergeSort(arr, mid + 1, high);
        merge(arr, low, mid, high);
    }
}

int main()
{
    int i;
    int arr[] = {6, 7, 2, 4, 9, 8};
    mergeSort(arr, 0, 5);

    printf("\n\n results: \n");
    for (i = 0; i < 6; i++)
    {
       printf("%d ", arr[i]);
    }
    printf("\n\n\n");

    return 0;
}

Answer 1

您确定要这样做吗？

i = 0;
j = 0;
k = 0;

乍一看，我认为应该是：

i = low;
j = 0;
k = 0;

Answer 2

Mergesort必须合并已排序的子序列：

a = 1,3,5
b = 2,4,6
c = merge(a, b) // C = 1,2,3,4,5,6

您没有对任何子序列进行排序。

Answer 3

问题在您的merge()函数中毫无疑问。

我认为合并功能会像这样（这里的任何错误留给读者：X ......这是未经测试的，不在袖口上，应仅用于概念目的！）：

/**
 * let mid be the last element of the first array
 * making mid + 1 the first element of the second
 */
merge(int[] arr, int low, int mid, int high) {

  int[] buff;
  int lowpos;
  int highpos;
  int i;

  buff = new int[high - low + 1];
  lowpos = low;
  highpos = mid + 1;

  // merge the two subarrays into the buffer
  for(i = 0; i < (high - low + 1); i++) {
    if(lowpos > mid)  buff[i] = arr[highpos++];

    else if(highpos > high) buff[i] = arr[lowpos++];

    else if(arr[lowpos] < arr[highpos] buff[i] = arr[lowpos++];

    else buff[i] = arr[highpos++];
  }

  // copy the buffer back into the main array
  for(i = 0; i < (high - low + 1) i++) arr[i+low] = buff[i];
}

有人能告诉我我的mergesort有什么问题吗？

3 个答案: