AtCoder Regular Contest 146
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Problem Statement
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The following process is carried out on a permutation $P$ of $(1,2,\dots,N)$.
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We have
$N$cards, numbered$1$to$N$. Card$i$has the integer$P_i$written on it.There are an integer
$X=1$and a boy called PCT, who initially has nothing. PCT does the following procedure for each$i=1,2,\dots,N$in this order.
- Get Card
$i$. Then, repeat the following action as long as he has a card with$X$written on it:- If PCT currently has
$M$or more cards, throw away all cards he has and terminate the process, without performing any more procedures.
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Here, let us define the score of the permutation $P$ as follows:
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- if the process is terminated by throwing away cards, the score of
$P$is$0$; - if the process is carried out through the end without throwing away cards, the score of
$P$is$\prod_{i=1}^{N-1}$$($the number of cards PCT has at the end of the$i$-th procedure$)$.
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There are $N!$ permutations $P$ of $(1,2,\dots,N)$. Find the sum of the scores of all those permutations, modulo $998244353$.
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Constraints
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$2 \le N \le 2 \times 10^5$$2 \le M \le N$- All values in input are integers.
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Input
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Input is given from Standard Input in the following format:
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N M
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Output
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Print the answer.
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样例
Input #1
3 2
Output #1
1
For $P=(3,1,2)$, the process goes as follows.
- The first procedure:
- PCT gets Card $1$.
- PCT currently has $1$ card, so he goes on.
- The second procedure:
- PCT gets Card $2$.
- PCT eats Card $2$ and make $X = 2$.
- PCT currently has $1$ card, so he goes on.
- The third procedure:
- PCT gets Card $3$.
- PCT eats Cards $1,3$ and make $X = 4$.
- PCT currently has $0$ cards, so he goes on.
The process is carried out through the end, so the score of $(3,1,2)$ is $1 \times 1 = 1$.
Other than $(3,1,2)$, there is no permutation with a score of $1$ or greater, so the answer is $1$.
Input #2
3 3
Output #2
5
Input #3
146146 146
Output #3
103537573
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。