pku 2299 Ultra-QuickSort 归并排序求逆序数

http://poj.org/problem?id=2299

归并虽然知道了过程但是敲起代码来还是出现各种小错误。。。唉，，，

#include <cstdio>
#include <iostream>
#define maxn 500007
using namespace std;

int a[maxn];
int n;
int tmp[maxn];
__int64 ct;

void merge(int s,int m,int e)
{
    int i = s,j = m + 1;
    int k = 0;
    while (i <= m && j <= e)
    {
        //s - m 和 m+1 —— e两个数组是已经排好序的
        //如果a[i] > a[j]则说明i - m的值都大于a[j]出现逆序数
        if (a[i] > a[j])
        {
            ct += m - i + 1;
            tmp[k++] = a[j++];
        }
        else
        {
            tmp[k++] = a[i++];
        }
    }
    while (i <= m)
    tmp[k++] = a[i++];
    while (j <= e)
    tmp[k++] = a[j++];
    for (i = 0; i < k; ++i)
    {
        a[s + i] = tmp[i];
    }
}
void mergesort(int s,int e)
{
    if (s < e)
    {
        int m = (s + e)/2;
        mergesort(s,m);
        mergesort(m + 1,e);
        merge(s,m,e);
    }
}
int main()
{
    int i;
    while (~scanf("%d",&n))
    {
        if (!n) break;
        ct = 0;
        for (i = 0; i < n; ++i)
        scanf("%d",&a[i]);
        mergesort(0,n - 1);
        printf("%I64d\n",ct);
    }
    return 0;
}