思路：
- 从头开始遍历，每遇到一个有child的节点，就将child的子链表嵌入原链表中即可，直至null。
public Node flatten(Node head) {
for (Node cursor = head; cursor != null; cursor = cursor.next) {
if (cursor.child != null) {
Node child = cursor.child; //取子节点
child.prev = cursor;
while (child.next != null) child = child.next; //移动到子节点末尾
if (cursor.next != null) cursor.next.prev = child;
child.next = cursor.next; //将这一小段链表嵌入原链表
cursor.next = cursor.child;
cursor.child = null;
}//重复上述操作，直至null，就连成一个链表了
}
return head;
}