AtCoder Regular Contest 154
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Problem Statement
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For a permutation $Q=(Q_1,Q_2,\dots,Q_N)$ of $(1,2,\dots,N)$, let $f(Q)$ be the following value:
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the sum of
j-i
over all pairs of integers
(i,j)
such that
1 \le i < j \le N
and
Q_i > Q_j
.
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You are given a permutation $P=(P_1,P_2,\dots,P_N)$ of $(1,2,\dots,N)$.
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Let us repeat the following operation $M$ times.
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- Choose a pair of integers
$(i,j)$such that$1 \le i \le j \le N$. Reverse$P_i,P_{i+1},\dots,P_j$. Formally, replace the values of$P_i,P_{i+1},\dots,P_j$with$P_j,P_{j-1},\dots,P_i$simultaneously.
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There are $\left(\frac{N(N+1)}{2}\right)^{M}$ ways to repeat the operation. Assume that we have computed $f(P)$ for all those ways.
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Find the sum of these $\left(\frac{N(N+1)}{2}\right)^{M}$ values, modulo $998244353$.
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Constraints
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$1 \le N,M \le 2 \times 10^5$$(P_1,P_2,\dots,P_N)$is a permutation of$(1,2,\dots,N)$.
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Input
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The input is given from Standard Input in the following format:
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N M
P_1 P_2 \dots P_N
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Output
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Print a single line containing the answer.
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样例
Input #1
2 1
1 2
Output #1
1
There are three ways to perform the operation, as follows.
- Choose $(i,j)=(1,1)$, making $P=(1,2)$, where $f(P)=0$.
- Choose $(i,j)=(1,2)$, making $P=(2,1)$, where $f(P)=1$.
- Choose $(i,j)=(2,2)$, making $P=(1,2)$, where $f(P)=0$.
Thus, the answer is $0+1+0=1$.
Input #2
3 2
3 2 1
Output #2
90
Input #3
10 2023
5 8 1 9 3 10 4 7 2 6
Output #3
543960046
题目信息
题面与样例版权归 AtCoder 及出题人所有;中文翻译由 AI 生成(CI 管线),如有出入请以原文为准。