poj2299解题报告（归并排序求逆序数）

//3880K	391MS
#include <cstdio>
#include <algorithm>

int buf[500000];
int ret[500000];
unsigned long long count;

void mergeArray(int *a, int first, int mid, int last, int *ret)
{
	int i = first, j = mid+1, k =0;
	int m = mid,   n = last;

	while(i<=m && j<=n){
		if (a[i] <= a[j]) {
			ret[k++] = a[i++];
		}
		else {
			ret[k++] = a[j++];
			count += m-i+1;   //记录
		}
	}

	while(i<=m) ret[k++] = a[i++];
	while(j<=n) ret[k++] = a[j++];

	for (int i=0;i<k; ++i) a[first+i] = ret[i];
}
void mergeSort(int data[], int first, int end, int *ret)
{
	if (first < end)
	{
		int mid = (first + end)/2;
		mergeSort(data, first, mid, ret);
		mergeSort(data, mid+1, end, ret);
		mergeArray(data, first, mid, end, ret);
	}
}
int main()
{
	int num;
	while (true)
	{
		scanf("%d", &num);
		if (num <= 0) break;

		for (int i=0; i<num; ++i) scanf("%d", &buf[i]);

		count = 0;
		mergeSort(buf, 0, num-1, ret);
		printf("%lld\n", count);
	}

	return 0;
}