Stack-based, inplace, recursive quicksort. I'm quite proud of this, I think it's quite clever. Odd numbers are sorted to the front, BTW.

epic_quicksort.cpp

int CompareOddEven(int lhs, int rhs)
{
	//lhs < rhs ? -1, 1, 0

	//odd vs even
	if (lhs % 2 == 1 && rhs % 2 == 0) {
		return -1;
	}
	if (lhs % 2 == 0 && rhs % 2 == 1) {
		return 1;
	}

	//size
	if (lhs < rhs) {
		return -1;
	}
	if (lhs > rhs) {
		return 1;
	}
	return 0;
}

inline void SwapReferences(int& lhs, int& rhs)
{
	//importantly, swapping the same value doesn't cause issues like XOR
	int tmp = lhs;
	lhs = rhs;
	rhs = tmp;
}

void SortOddEven(int* numbers, int count)
{
	//error checking
	if (!numbers || count < 2) {
		return;
	}

	//stack-based, inplace, recursive quicksort FTW
	int pivot = count / 2;
	int stack = pivot;
	int iter = pivot;

	//left of the pivot
	while (--iter >= 0) {
		if (CompareOddEven(numbers[iter], numbers[pivot]) > 0) {
			//push to the stack
			SwapReferences(numbers[iter], numbers[--stack]);
		}
	}
	//move the stack
	SwapReferences(numbers[stack], numbers[pivot]);
	SwapReferences(stack, pivot);

	//reset, skipping the pre-existing stack in the next loop
	iter = stack;
	stack = pivot;

	//right of the pivot
	while(++iter < count) {
		if (CompareOddEven(numbers[iter], numbers[pivot]) < 0) {
			//push to the stack
			SwapReferences(numbers[iter], numbers[++stack]);
		}
	}
	//move the stack
	SwapReferences(numbers[stack], numbers[pivot]);
	SwapReferences(stack, pivot);

	//recursive calls
	SortOddEven(numbers, pivot);
	SortOddEven(numbers + pivot + 1, count - pivot - 1);
}