大和証券プログラミングコンテスト2021 (AtCoder Regular Contest 128)
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Problem Statement
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We have $N$ balls arranged in a row, numbered $1$ to $N$ from left to right. Ball $i$ has an integer $A_i$ written on it.
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You can do the following operation any number of times.
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- Choose three consecutive balls
$x, y, z$$(1 \leq x < y < z \leq N$). Here,$A_x \neq A_y$and$A_y \neq A_z$must hold. Then, eat Ball$y$. After this operation, Balls$x$and$z$are considered adjacent.
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Find the number of possible sets of balls remaining in the end, modulo $998244353$.
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Constraints
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$2 \leq N \leq 200000$$1 \leq A_i \leq 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
A_1 A_2 \cdots A_N
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Output
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Print the answer.
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样例
Input #1
4
1 2 1 2
Output #1
3
There are three possible sets of balls remaining in the end: $\{1,2,3,4\},\{1,2,4\},\{1,3,4\}$.
Input #2
5
5 4 3 2 1
Output #2
8
Different sequences of operations are not distinguished if they result in the same set of balls.
Input #3
5
1 2 3 2 1
Output #3
8
Different sets of remaining balls are distinguished even if they have the same sequence of integers written on them.
Input #4
9
1 4 2 2 9 6 9 6 6
Output #4
14
题目信息
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