QuickSort（分区Hoare）的修改，先偶数降序，再奇数降序

Question

I have a big problem to modify the Hoare partition so that it sorts in descending order: first by even numbers and then by odd numbers. Example: arr[] = {1, 6, 7, 8, 4, 5}, out: arr[] = {8, 6, 4, 7, 5, 1}.

I was able to do this by dividing the array separately into even and odd numbers and sorting each of these parts separately. However, the task is to rework the quicksort so that you do not have to divide the array.

Below I have my partition, I'm going in the right direction?

static int partition(int []arr, int low,  
                                int high) 
{ 
    int pivot = arr[low]; 
    int i = low - 1, j = high + 1; 

    while (true) 
    { 

        do
        { 
            i++; 
        } while ((arr[i] > pivot && ((arr[i]%2==0 && pivot%2==0) || (arr[i]%2!=0 && pivot%2!=0))) || (arr[i]<pivot && arr[i]%2==0 && pivot%2!=0)); 


        do
        { 
            j--; 
        } while (arr[j] < pivot); 

        if (i >= j) 
            return j; 
        int temp = arr[i]; 
        arr[i] = arr[j]; 
        arr[j] = temp; 
    } 
} 

Answer 1

You did an effort in the condition of the first loop, but:

• It is not entirely correct
• It should be applied similarly to the second loop condition as well

For the second reason it may be good to define a compare-function:

    private static int cmp(int a, int b) {
        if (a % 2 != b % 2) return a % 2 - b % 2; 
        return b - a;
    }

Now the loop conditions should be simple:

• while (cmp(arr[i], pivot) < 0)
• while (cmp(arr[j], pivot) > 0)