归并排序求逆序数

归并排序求逆序数

转自：http://www.blog.edu.cn/user2/sempr/archives/2006/1142716.shtml
#include<cstdio>

const int MAX = 10000;
int a[MAX],b[MAX];
int change;
void merge(int p, int q, int r)
{
	int i, j = 0;
	int begA = p, endA = q, begB = q+1, endB = r;
    while(begA <= endA && begB <= endB)
	{
		if(a[begA] <= a[begB])
			b[j++] = a[begA++];
		else
		{
			b[j++] = a[begB++];
			change += q - begA + 1;
        }
	}
	while(begA <= endA)
		b[j++] = a[begA++];
    while(begB <= endB)
		b[j++] = a[begB++];
    for(i = 0; i < j; i++)
		a[p+i] = b[i];
}
void mergeSort(int first, int last)
{
	if(first < last)
	{
		int mid = (first + last) / 2;
		mergeSort(first, mid);
        mergeSort(mid+1, last);
        merge(first, mid, last);
    }
}
int main()
{
	return 0;
}