パナソニックプログラミングコンテスト2021(AtCoder Beginner Contest 231)
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Problem Statement
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We have $N$ boxes numbered $1$ to $N$. Initially, Box $i$ contains $A_i$ balls.
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You will repeat the following operation $K$ times.
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- Choose one box out of the
$N$uniformly at random (each time independently). Add one ball to the chosen box.
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Let $B_i$ be the number of balls in Box $i$ after the $K$ operations, and the score be the product of the numbers of balls, $\prod_{i=1}^{N}B_i$.
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Find the expected value of the score modulo $998244353$.
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Note
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When the expected value in question is represented as an irreducible fraction $p/q$, there uniquely exists an integer $r$ such that $rq\equiv p \pmod{998244353}$ and $0\leq r < 998244353$ under the Constraints of this problem. This $r$ is the value we seek.
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Constraints
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$1 \leq N \leq 1000$$1 \leq K \leq 10^9$$0 \leq A_i \leq 10^9$
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Input
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Input is given from Standard Input in the following format:
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N K
A_1 \ldots A_N
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Output
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Print the answer.
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样例
Input #1
3 1
1 2 3
Output #1
665496245
After the operation, the score will be as follows.
- When choosing Box $1$ in the operation, $2\times 2\times 3=12$.
- When choosing Box $2$ in the operation, $1\times 3\times 3=9$.
- When choosing Box $3$ in the operation, $1\times 2\times 4=8$.
Therefore, the expected value in question is $\frac{1}{3}(12+9+8)=\frac{29}{3}$. This value modulo $998244353$ is $665496245$.
Input #2
2 2
1 2
Output #2
499122182
After the operations, the score will be as follows.
- When choosing Box $1$ in the first operation and Box $1$ in the second, $3\times 2=6$.
- When choosing Box $1$ in the first operation and Box $2$ in the second, $2\times 3=6$.
- When choosing Box $2$ in the first operation and Box $1$ in the second, $2\times 3=6$.
- When choosing Box $2$ in the first operation and Box $2$ in the second, $1\times 4=4$.
Therefore, the expected value in question is $\frac{1}{4}(6+6+6+4)=\frac{11}{2}$.
Input #3
10 1000000000
998244350 998244351 998244352 998244353 998244354 998244355 998244356 998244357 998244358 998244359
Output #3
138512322
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。