O(nlogn)
在归并过程中得到答案

ll merge_sort(int a[], int l, int r)
{
    if (l >= r)
    {
        return 0;
    }
    int mid = l + r >> 1;
    ll cnt = merge_sort(a, l, mid) + merge_sort(a, mid + 1, r);
    int k = 0, i = l, j = mid + 1;
    while (i <= mid && j <= r)
    {
        if (a[i] <= a[j])
        {
            tmp[k++] = a[i++];
        }
        else
        {
            tmp[k++] = a[j++];
            cnt += mid - i + 1;
        }
    }
    while (i <= mid)
    {
        tmp[k++] = a[i++];
    }
    while (j <= r)
    {
        tmp[k++] = a[j++];
    }
    for (i = l, j = 0; i <= r;)
    {
        a[i++] = tmp[j++];
    }
    return cnt;
}