求逆序对数（利用归并排序）

int cnt = 0;
void merge_sort(int *A, int x, int y, int *T)
{
if(y-x > 1)
{
int m = x + (y-x)/2;
int p = x, q = m, i = x;
merge_sort(A, x, m, T);
merge_sort(A, m, y, T);
while(p < m || q < y)
{
if(q >= y || (p < m && A[p] <= A[q]))
T[i++] = A[p++];
else
{
T[i++] = A[q++];
cnt += m - p;//此为修改后；
}
}
for(i = x; i < y; i++)
A[i] = T[i];
}
}