B. Emordnilap

B. Emordnilap

A permutation of length $n$ is an array consisting of $n$ distinct integers from $1$ to $n$ in arbitrary order. For example, $[2,3,1,5,4]$ is a permutation, but $[1,2,2]$ is not a permutation ($2$ appears twice in the array), and $[1,3,4]$ is also not a permutation ($n=3$ but there is $4$ in the array). There are $n! = n \cdot (n-1) \cdot (n - 2) \cdot \ldots \cdot 1$ different permutations of length $n$.

Given a permutation $p$ of $n$ numbers, we create an array $a$ consisting of $2n$ numbers, which is equal to $p$ concatenated with its reverse. We then define the beauty of $p$ as the number of inversions in $a$.

The number of inversions in the array $a$ is the number of pairs of indices $i$, $j$ such that $i < j$ and $a_i > a_j$.

For example, for permutation $p = [1, 2]$, $a$ would be $[1, 2, 2, 1]$. The inversions in $a$ are $(2, 4)$ and $(3, 4)$ (assuming 1-based indexing). Hence, the beauty of $p$ is $2$.

Your task is to find the sum of beauties of all $n!$ permutations of size $n$. Print the remainder we get when dividing this value by $1\,000\,000\,007$ ($10^9 + 7$).

Input

Each test contains multiple test cases. The first line contains the number of test cases $t$ ($1 \le t \le 10^5$). The description of the test cases follows.

Each test case has only one line — the integer $n$ ($1 \leq n \leq 10^5$).

It is guaranteed that the sum of $n$ over all test cases does not exceed $10^5$.

Output

For each test case, print one integer — the sum of beauties of all permutations of size $n$ modulo $1\,000\,000\,007$ ($10^9 + 7$).

Example

input

3
1
2
100

output

0
4
389456655

Note

For the first test case of the example, $p = [1]$ is the only permutation. $a = [1, 1]$ has $0$ inversions.

For the second test case of the example, the permutations are $[1, 2]$ and $[2, 1]$. Their respective $a$ arrays are $[1, 2, 2, 1]$ and $[2, 1, 1, 2]$, both of which have $2$ inversions.

 

解题思路

  比赛的时候想不出正解,然后模拟了几个样例找规律,发现任意一个排列逆序对的数量都相同,均是$n(n-1)$,因为一共有$n!$种排列,因此答案就是$n(n-1) \cdot n!$。

  参考官方题解的推导。考虑排列中的两个下标$i$和$j$($i < j$),在任意一个排列中他们形式一定是$[\dots p_i \ldots p_j \ldots p_j \ldots p_i \ldots]$ ,然后分情况讨论:

  • $p_i > p_j$。那么排列中$p_i$对于$p_j$而言就存在$2$个逆序对。
  • $p_i < p_j$。那么排列中$p_j$对于$p_i$而言就存在$2$个逆序对。

  因此对于任意一对下标$i, j$都会存在两个逆序对,而一个排列中共有$C_{n}^{2}$对这样的下标,因此一个排列中的逆序对个数就是$2 \cdot C_{n}^{2}$。

  由于有$n!$种不同的排列,故总的逆序对个数就是$2 \cdot C_{n}^{2} \cdot n! = n(n-1) \cdot n!$。

  AC代码如下:

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 
 4 typedef long long LL;
 5 
 6 const int mod = 1e9 + 7;
 7 
 8 void solve() {
 9     int n;
10     scanf("%d", &n);
11     int ret = (n - 1ll) * n % mod;
12     for (int i = 1; i <= n; i++) {
13         ret = 1ll * ret * i % mod;
14     }
15     printf("%d\n", ret);
16 }
17 
18 int main() {
19     int t;
20     scanf("%d", &t);
21     while (t--) {
22         solve();
23     }
24     
25     return 0;
26 }

 

参考资料

  Codeforces Round #845 (Div. 2) and ByteRace 2023 Editorial:https://codeforces.com/blog/entry/111729

原文链接: https://www.cnblogs.com/onlyblues/p/17064291.html

欢迎关注

微信关注下方公众号,第一时间获取干货硬货;公众号内回复【pdf】免费获取数百本计算机经典书籍

    B. Emordnilap

原创文章受到原创版权保护。转载请注明出处:https://www.ccppcoding.com/archives/313602

非原创文章文中已经注明原地址,如有侵权,联系删除

关注公众号【高性能架构探索】,第一时间获取最新文章

转载文章受原作者版权保护。转载请注明原作者出处!

(0)
上一篇 2023年2月16日 下午12:58
下一篇 2023年2月16日 下午12:58

相关推荐