even odd sort (C)

給你一個n，接下來有n個整數，對他們排序使得
(1) 偶數在奇數前面
(2) 偶數遞減排序、奇數遞增排序
然後輸出這個數列

// Comment
#include <stdio.h>
#include <stdlib.h>

int lessthan(const void * p, const void * q){
  return *(int*)p - *(int*)q;
}

int greaterthan(const void * p, const void * q){
  return *(int*)q - *(int*)p;
}

void eosort(int * arr, int arrSize){
  int *a1 = (int*)malloc(sizeof(int)*arrSize);
  int *a2 = (int*)malloc(sizeof(int)*arrSize);
  int odd_index = 0;
  int even_index = 0;
  int i;
  for(i = 0; i < arrSize; ++i){
    if(arr[i] % 2){
      a1[odd_index++] = arr[i];
    }
    else
      a2[even_index++] = arr[i];
  }
  qsort(a1, odd_index, sizeof(int), lessthan);
  qsort(a2, even_index, sizeof(int), greaterthan);

  for(i = 0; i < even_index; ++i){
    arr[i] = a2[i];
  }
  for(i = 0; i < odd_index; ++i){
    arr[i + even_index] = a1[i];
  }
}


int main(void){
  int arr[] = {0, 9, 2, 7, 4, 5, 6, 3, 8, 1, 10};
  int arrSize = 11;
  eosort(arr, arrSize);
  int i;
  for(i = 0; i < arrSize; ++i){
    printf("%d ", arr[i]);
  }
  putchar('\n');
  return 0;
}