Abnormal Permutation Pairs (hard version)
题意翻译
给定 $n,m$,求有多少对长度为 $n$ 的排列 $p,q$,满足以下条件。
- $p$ 的字典序小于 $q$
- $p$ 的逆序对个数 **严格大于** $q$.
答案对 $m$ 取模。
$1\le n\le 500,1\le m\leq 10^9$,**不保证** $m$ 为素数。
题目描述
This is the hard version of the problem. The only difference between the easy version and the hard version is the constraints on $ n $ . You can only make hacks if both versions are solved.
A permutation of $ 1, 2, \ldots, n $ is a sequence of $ n $ integers, where each integer from $ 1 $ to $ n $ appears exactly once. For example, $ [2,3,1,4] $ is a permutation of $ 1, 2, 3, 4 $ , but $ [1,4,2,2] $ isn't because $ 2 $ appears twice in it.
Recall that the number of inversions in a permutation $ a_1, a_2, \ldots, a_n $ is the number of pairs of indices $ (i, j) $ such that $ i < j $ and $ a_i > a_j $ .
Let $ p $ and $ q $ be two permutations of $ 1, 2, \ldots, n $ . Find the number of permutation pairs $ (p,q) $ that satisfy the following conditions:
- $ p $ is lexicographically smaller than $ q $ .
- the number of inversions in $ p $ is greater than the number of inversions in $ q $ .
Print the number of such pairs modulo $ mod $ . Note that $ mod $ may not be a prime.
输入输出格式
输入格式
The only line contains two integers $ n $ and $ mod $ ( $ 1\le n\le 500 $ , $ 1\le mod\le 10^9 $ ).
输出格式
Print one integer, which is the answer modulo $ mod $ .
输入输出样例
输入样例 #1
4 403458273
输出样例 #1
17
说明
The following are all valid pairs $ (p,q) $ when $ n=4 $ .
- $ p=[1,3,4,2] $ , $ q=[2,1,3,4] $ ,
- $ p=[1,4,2,3] $ , $ q=[2,1,3,4] $ ,
- $ p=[1,4,3,2] $ , $ q=[2,1,3,4] $ ,
- $ p=[1,4,3,2] $ , $ q=[2,1,4,3] $ ,
- $ p=[1,4,3,2] $ , $ q=[2,3,1,4] $ ,
- $ p=[1,4,3,2] $ , $ q=[3,1,2,4] $ ,
- $ p=[2,3,4,1] $ , $ q=[3,1,2,4] $ ,
- $ p=[2,4,1,3] $ , $ q=[3,1,2,4] $ ,
- $ p=[2,4,3,1] $ , $ q=[3,1,2,4] $ ,
- $ p=[2,4,3,1] $ , $ q=[3,1,4,2] $ ,
- $ p=[2,4,3,1] $ , $ q=[3,2,1,4] $ ,
- $ p=[2,4,3,1] $ , $ q=[4,1,2,3] $ ,
- $ p=[3,2,4,1] $ , $ q=[4,1,2,3] $ ,
- $ p=[3,4,1,2] $ , $ q=[4,1,2,3] $ ,
- $ p=[3,4,2,1] $ , $ q=[4,1,2,3] $ ,
- $ p=[3,4,2,1] $ , $ q=[4,1,3,2] $ ,
- $ p=[3,4,2,1] $ , $ q=[4,2,1,3] $ .