AtCoder Regular Contest 147
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Problem Statement
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Consider a sequence of integer sets of length $N$: $S=(S_1,S_2,\dots,S_N)$. We call a sequence brilliant if it satisfies all of the following conditions:
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$S_i$is a (possibly empty) integer set, and its elements are in the range$[1,M]$.$(1 \le i \le N)$- The number of integers that is included in exactly one of
$S_i$and$S_{i+1}$is$1$.$(1 \le i \le N-1)$
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We define the score of a brilliant sequence $S$ as $\displaystyle \prod_{i=1}^{M}$ $($ the number of sets among $S_1,S_2,\dots,S_N$ that include $i$.$)$.
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Find, modulo $998244353$, the sum of the scores of all possible brilliant sequences.
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Constraints
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$1 \le N,M \le 2 \times 10^5$- 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
2 3
Output #1
24
Among all possible brilliant sequences, the following $6$ have positive scores.
- $S_1=\{1,2\},S_2=\{1,2,3\}$
- $S_1=\{1,3\},S_2=\{1,2,3\}$
- $S_1=\{2,3\},S_2=\{1,2,3\}$
- $S_1=\{1,2,3\},S_2=\{1,2\}$
- $S_1=\{1,2,3\},S_2=\{1,3\}$
- $S_1=\{1,2,3\},S_2=\{2,3\}$
All of them have a score of $4$, so the sum of them is $24$.
Input #2
12 34
Output #2
786334067
题目信息
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