Codeforces Round 902 (Div. 1, based on COMPFEST 15 - Final Round)
D
Lexichromatography
Codeforces
2500
原题
← CE →
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Pak Chanek loves his faculty, the Faculty of Computer Science, University of Indonesia (Fasilkom). He wants to play with the colours of the faculty's logo, blue and red.

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There is an array $a$ consisting of $n$ elements, element $i$ has a value of $a_i$. Pak Chanek wants to colour each element in the array blue or red such that these following conditions are satisfied:

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  • If all blue elements are formed into a subsequence$^\dagger$ and so are all the red elements, the blue subsequence is strictly less than the red subsequence lexicographically$^\ddagger$.
  • Array $a$ does not have any subarray that is imbalanced. A subarray is imbalanced if and only if there is a value $k$ such that the absolute difference between the number of blue elements with value $k$ and the number of red elements with value $k$ in this subarray is $2$ or more.
  • Note that it is possible to colour every element of the array the same colour.
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How many different colourings satisfy all those conditions? Since the answer can be very big, print the answer modulo $998\,244\,353$. Two colourings are different if and only if there is at least one element that is blue in one colouring, but red in the other.

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$^\dagger$ A subsequence of an array is a sequence that can be obtained from the array by deleting some elements (possibly none), without changing the order of the remaining elements.

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$^\ddagger$ Let $p$ and $q$ be two different sequences. Sequence $p$ is said to be lexicographically less than sequence $q$ if and only if $p$ is a prefix of $q$ or there is an index $i$ such that $p_j=q_j$ holds for every $1\leq j \lt i$, and $p_i \lt q_i$. In particular, an empty sequence is always lexicographically less than any non-empty sequence.

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Input

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The first line contains a single integer $n$ ($1 \leq n \leq 2\cdot10^5$) — the size of array $a$.

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The second line contains $n$ integers $a_1,a_2,a_3,\ldots,a_n$ ($1\leq a_i\leq2\cdot10^5$).

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Output

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An integer representing the number of different colourings that satisfy all of the problem's conditions, modulo $998\,244\,353$.

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Note

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In the first example, the $3$ ways for colouring all elements from index $1$ to index $8$ are:

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  • Blue, red, red, blue, blue, red, red, blue.
  • Blue, red, red, red, blue, blue, red, blue.
  • Red, red, blue, blue, blue, red, red, blue.
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As an example, if we colour the elements from index $1$ to index $8$ to be red, red, blue, red, red, blue, blue, blue, it is not valid colouring, because for subarray $a[2..6]$, there are $0$ blue elements with value $3$ and $2$ red elements with value $3$, making subarray $a[2..6]$ an imbalanced subarray.

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样例

Input #1
8
1 3 1 2 3 2 3 3
Output #1
3
InputCopy OutputCopy
Input #2
1
265
Output #2
1
InputCopy OutputCopy
题目信息
题号
1876D
时间限制
2 s
内存限制
512 MB
标签
组合数学
DFS 及类似
并查集
图论
双指针

题面与样例版权归 Codeforces 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。