POJ 2299 Ultra-QuickSort

#include <iostream>
#include <cstdlib>
#include <cstring>
#include <iomanip>
#include <cstdio>
#include <cmath>

using namespace std;

long long a[500002], b[500002], cct;
void merge_sort(int x, int y)
{
if(y - x > 1)
{
int m = x + (y - x) / 2;
int xx = x, yy = m, i = x;
merge_sort(x, m);
merge_sort(m, y);
while(xx < m || yy < y)
{
if(yy >= y || (xx < m && a[xx] <= a[yy]))
b[i++] = a[xx++];
else
{
b[i++] = a[yy++];
cct += m - xx;
}
}
for(int i = x; i < y; i++)
a[i] = b[i];
}
}
int main()
{
#ifdef test
freopen("in.txt", "r", stdin);
#endif
int num;
while(scanf("%d", &num), num)
{
for(int i = 0; i < num; i++)
scanf("%I64d", &a[i]);
cct = 0;
merge_sort(0, num);
printf("%I64d\n", cct);
}
return 0;
}