逆序数

#include <stdio.h>
#include <string.h>

int InversionPair(int * arr, int low, int high, int * assist)
{
	if(low >= high)
		return 0;
	int mid = low + (high - low) / 2;
	int lRet = InversionPair(arr, low, mid, assist);
	int rRet = InversionPair(arr, mid + 1, high, assist);	

	int p = low, q = mid + 1;
	int k = low;
	int mRet = 0;

	while(p <= mid || q <= high)
	{
		if(((p <= mid || q <= high) && arr[p] > arr[q]) || q > high)		
			assist[k++] = arr[p++];
		else	
		{
			assist[k++] = arr[q++];		
			mRet += (p - low);
		}
	}
	memcpy(arr + low, assist + low, (high - low + 1)*sizeof(int));

	return lRet + rRet + mRet;
}

int main()
{
	int arr[5] = {4,-1,3,2,0};
	int assist[5];
	int inverPairCount = InversionPair(arr, 0, 4, assist);
	printf("%d\n", inverPairCount);
	for(int i = 0; i < sizeof(arr) / sizeof(int); i++)
		printf("%d ", arr[i]);
	printf("\n");
}