You are an ant tasked with adding n new rooms numbered 0 to n-1 to your colony. You are given the expansion plan as a 0-indexed integer array of length n, prevRoom, where prevRoom[i] indicates that you must build room prevRoom[i] before building room i, and these two rooms must be connected directly. Room 0 is already built, so prevRoom[0] = -1. The expansion plan is given such that once all the rooms are built, every room will be reachable from room 0.
You can only build one room at a time, and you can travel freely between rooms you have already built only if they are connected. You can choose to build any room as long as its previous room is already built.
\r\n\r\nReturn the number of different orders you can build all the rooms in. Since the answer may be large, return it modulo 109 + 7.
\r\n
Example 1:
\r\n![\"\"](\"https://assets.leetcode.com/uploads/2021/06/19/d1.JPG\")
\r\n\r\nInput: prevRoom = [-1,0,1]\r\nOutput: 1\r\nExplanation: There is only one way to build the additional rooms: 0 → 1 → 2\r\n\r\n\r\n
Example 2:
\r\n![\"\"](\"https://assets.leetcode.com/uploads/2021/06/19/d2.JPG\")
\r\n\r\n\r\nInput: prevRoom = [-1,0,0,1,2]\r\nOutput: 6\r\nExplanation:\r\nThe 6 ways are:\r\n0 → 1 → 3 → 2 → 4\r\n0 → 2 → 4 → 1 → 3\r\n0 → 1 → 2 → 3 → 4\r\n0 → 1 → 2 → 4 → 3\r\n0 → 2 → 1 → 3 → 4\r\n0 → 2 → 1 → 4 → 3\r\n\r\n\r\n
\r\n
Constraints:
\r\n\r\nn == prevRoom.length2 <= n <= 105prevRoom[0] == -10 <= prevRoom[i] < n for all 1 <= i < n0 once all the rooms are built.你是一只蚂蚁，负责为蚁群构筑 n 间编号从 0 到 n-1 的新房间。给你一个 下标从 0 开始 且长度为 n 的整数数组 prevRoom 作为扩建计划。其中，prevRoom[i] 表示在构筑房间 i 之前，你必须先构筑房间 prevRoom[i] ，并且这两个房间必须 直接 相连。房间 0 已经构筑完成，所以 prevRoom[0] = -1 。扩建计划中还有一条硬性要求，在完成所有房间的构筑之后，从房间 0 可以访问到每个房间。
你一次只能构筑 一个 房间。你可以在 已经构筑好的 房间之间自由穿行，只要这些房间是 相连的 。如果房间 prevRoom[i] 已经构筑完成，那么你就可以构筑房间 i。
返回你构筑所有房间的 不同顺序的数目 。由于答案可能很大，请返回对 109 + 7 取余 的结果。
\n\n
示例 1：
\n\n\n输入：prevRoom = [-1,0,1]\n输出：1\n解释：仅有一种方案可以完成所有房间的构筑：0 → 1 → 2\n\n\n示例 2：
\n\n\n\n输入：prevRoom = [-1,0,0,1,2]\n输出：6\n解释：\n有 6 种不同顺序：\n0 → 1 → 3 → 2 → 4\n0 → 2 → 4 → 1 → 3\n0 → 1 → 2 → 3 → 4\n0 → 1 → 2 → 4 → 3\n0 → 2 → 1 → 3 → 4\n0 → 2 → 1 → 4 → 3\n\n\n\n\n
提示：
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