归并排序求逆序数

#include <cstdio>  
  
int left[250003], right[250003];  
__int64 count;  
  
void merge(int a[], int l, int m, int r)  
{  
    int i, j, k, n1, n2;  
  
    n1 = m - l + 1;  
    n2 = r - m;  
    for (i = 0; i < n1; i++)  
        left[i] = a[l+i];  
    for (i = 0; i < n2; i++)  
        right[i] = a[m+i+1];  
    left[n1] = right[n2] = 0x7fffffff;  
  
    i = j = 0;  
    for (k = l; k <= r; k++)  
    {  
        if (left[i] <= right[j])  
        {  
            a[k] = left[i++];  
        }  
        else  
        {  
            a[k] = right[j++];  
            count += n1 - i;   
        }  
    }  
}  
  
void mergeSort(int a[], int l, int r)  
{  
    if (l < r)  
    {  
        int m = (l + r) / 2;  
        mergeSort(a, l, m);  
        mergeSort(a, m+1, r);  
        merge(a, l, m, r);  
    }  
}  
  
int main()  
{  
    int n, a[500001];  
  
    while (scanf("%d", &n) && n)  
    {  
        count = 0;  
        for (int i = 0; i < n; i++)  
            scanf("%d", &a[i]);  
        mergeSort(a, 0, n-1);  
        printf("%I64d\n", count);  
    }  
}