POJ 2299 Ultra-QuickSort

经典问题，归并排序，求逆序数。

代码如下：

#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;
}