poj 2299 求逆序数

#include <iostream>
const int MAX = 500000;
int a[MAX];
int swap[MAX];        //临时数组
int n;                //数组a的长度
__int64 result;            //数组a中的逆序数

//归并两个已经有序的段：a[low]—a[mid]和a[mid+1]—a[high]，使得a[low]—a[high]有序。
void merge(int low, int mid, int high)
{
    int i = low;
    int j = mid + 1;
    int m = 0;
    while(i <= mid && j <= high)
    {
        if(a[i] <= a[j])
        {
            swap[m++] = a[i];
            i++;
        }
        else
        {
            swap[m++] = a[j];
            j++;
            result += mid - i + 1;
        }
    }
    while(i <= mid)
    {
        swap[m++] = a[i];
        i++;
    }
    while(j <= mid)
    {
        swap[m++] = a[j];
        j++;
    }
    for(i = 0; i < m; i++)
        a[low + i] = swap[i];
}

//归并排序：对a[low]—a[high]进行归并排序。
void mergeSort(int low, int high)
{
    int mid;
    if(low < high)
    {
        mid = (low + high) /2;
        mergeSort(low, mid);
        mergeSort(mid + 1, high);
        merge(low, mid, high);
    }
}

int main()
{
    int i;
    while(true)
    {
        scanf("%d",&n);
        if(n == 0) break;
        result = 0;
        for(i = 0; i < n; i++)
            scanf("%d",a+i);
        mergeSort(0, n-1);
        printf("%I64d\n",result);
    }
    return 0;
}