逆序对的数量

利用归并排序操作

void merge_sort(int q[], int l, int r)
{
    if (l >= r) return;
    int mid = l + r >> 1;
    merge_sort(q, l, mid), merge_sort(q, mid + 1, r);
    int i = l, j = mid + 1,k = 0;

    while (i <= mid && j <= r)//合并操作，左右两边已经排好序
    {
        if (q[i] <= q[j])
        {
            cmp[k ++] = q[i ++];
        }
        else
        {
            res += mid - i + 1;//当左边有个数下标i比右边的一个数大，那有mid - i + 1个逆序对
            cmp[k ++] = q[j ++];
        }
    }

    while (i <= mid) cmp[k ++] = q[i ++];
    while (j <= r) cmp[k ++] = q[j ++];

    for (int i = l, j = 0; i <= r; i ++, j ++) q[i] = cmp[j];
}