POJ 2299 Ultra-QuickSort 归并排序

#include <cstdio>

int a[500000], b[500000];
long long cnt, n;

void mergesort ( int l, int r )
{
if ( l >= r ) return;

int mid = ( l + r ) / 2;
mergesort(l,mid);
mergesort(mid+1,r);

int i = l, j = mid+1, k = l;
while ( i <= mid && j <= r )
{
if ( a[i] > a[j] )
{
b[k++] = a[j++];
cnt += mid-i+1;/* a[i] > a[j], 表示出现了逆序对，此时由于a[i..m]是已经有序了，那么a[i+1], a[i+2], ... a[m]都是大于a[j]的， 都可以和a[j]组成逆序对，因此number += m - i + 1 */
}
else
b[k++] = a[i++];
}
while ( i <= mid ) b[k++] = a[i++];
while ( j <= r ) b[k++] = a[j++];
for ( i = l; i <= r; i++ )
a[i] = b[i];
}

int main()
{
while ( scanf("%d",&n) && n != 0 )
{
for ( int i = 0; i < n; i++ )
scanf("%d",&a[i]);
cnt = 0;
mergesort(0,n-1);
printf("%lld\n",cnt);
}
return 0;
}