poj 2299 求逆序数

 #include <iostream>
 const int MAX = ;
 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 + ;
     int m = ;
     while(i <= mid && j <= high)
     {
         if(a[i] <= a[j])
         {
             swap[m++] = a[i];
             i++;
         }
         else
         {
             swap[m++] = a[j];
             j++;
             result += mid - i + ;
         }
     }
     while(i <= mid)
     {
         swap[m++] = a[i];
         i++;
     }
     while(j <= mid)
     {
         swap[m++] = a[j];
         j++;
     }
     for(i = ; 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) /;
         mergeSort(low, mid);
         mergeSort(mid + , high);
         merge(low, mid, high);
     }
 }

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