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No.827 総神童数

レベル : / 実行時間制限 : 1ケース 2.000秒 / メモリ制限 : 512 MB / 標準ジャッジ問題
タグ : (AC するまで非表示) / 解いたユーザー数 137
作問者 : Pachicobue / テスター : kopricky
お気に入りにしたユーザー ProblemId : 2978 / 自分の提出
問題文最終更新日: 2019-05-02 18:41:29
Total Prodigy Count
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Problem Statement

Aoki has obtained a rooted tree $T$ consisting of vertices $1,2,...,N$. The root of $T$ is vertex $1$.
Aoki is considering writing the values $1,2,...,N$ on the vertices of the rooted tree $T$, without duplicates.
That is, letting $c_i$ be the value written on vertex $i$, the sequence $\{c_i\}$ must be a permutation of $\{1,2,...,N\}$.

Aoki has decided to call vertex $i$ a prodigy when "vertex $i$ has a value written on it that is greater than the values of all ancestors of vertex $i$ (excluding itself)". The root is always a prodigy.
He also defined the prodigy count as the number of vertices among the $N$ vertices that are prodigies.
For example, for the assignment of values shown in the figure below, the yellow vertices are prodigies, and the prodigy count is 4.
Here, the vertex with the value $6$ written on it is a prodigy, because the values $\{3,1\}$ are written on its ancestors, which contain only values less than $6$.
The vertex with the value $4$ written on it is not a prodigy, because the values $\{3,7\}$ are written on its ancestors, which contain a value greater than $4$.

Aoki became curious about how excellent the rooted tree $T$ is, and asked what the total of the prodigy counts would be if all $N!$ ways of writing the values were tried.
Your task is to compute this answer on Aoki's behalf. Since this value can be very large, report it modulo $10^9+7$.

Input

$N$ 
$u_1\ v_1$
$u_2\ v_2$
$\vdots$
$u_{N-1}\ v_{N-1}$

The first line contains the number of vertices $N$ of the tree.
Each of the 2nd through $N$-th lines contains space-separated integers $u, v$ representing the endpoints of an edge of the tree $T$. This means that vertices $u, v$ are connected bidirectionally.

Constraints
  • $1 \le N \le 2\times 10^5$
  • $1 \le u_i,v_i \le N$
  • The input graph is a tree with $N$ vertices.
  • All input values are integers.

Output

Output on one line the sum of the prodigy counts over all $N!$ ways of writing values on $T$, modulo $10^9+7$.
Print a newline at the end.

Sample

Sample 1
Input
2
1 2
Output
3

There are the following 2 ways of writing the values, and the yellow vertices are prodigies.
On the right, the vertex with 1 written on it is not a prodigy, because its parent's value is 2, which is greater than its own value.

Sample 2
Input
3
1 2
2 3
Output
11

There are the following 6 ways of writing the values, and the yellow vertices are prodigies.

Sample 3
Input
20
2 15
6 5
12 1
7 9
17 2
15 5
2 4
17 16
12 2
8 17
17 19
18 11
20 8
20 3
13 9
11 10
11 20
14 8
11 7
Output
820010006

Note that the answer is taken modulo $10^9+7$.

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