Codeforces Round 778 (Div. 1 + Div. 2, based on Technocup 2022 Final Round)
H
Three Minimums
Codeforces
3500
原题
← G→
翻译中…

Given a list of distinct values, we denote with first minimum, second minimum, and third minimum the three smallest values (in increasing order).

翻译中…

A permutation $p_1, p_2, \dots, p_n$ is good if the following statement holds for all pairs $(l,r)$ with $1\le l \lt l+2 \le r\le n$.

翻译中…
  • If $\{p_l, p_r\}$ are (not necessarily in this order) the first and second minimum of $p_l, p_{l+1}, \dots, p_r$ then the third minimum of $p_l, p_{l+1},\dots, p_r$ is either $p_{l+1}$ or $p_{r-1}$.
翻译中…

You are given an integer $n$ and a string $s$ of length $m$ consisting of characters "<" and ">".

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Count the number of good permutations $p_1, p_2,\dots, p_n$ such that, for all $1\le i\le m$,

翻译中…
  • $p_i \lt p_{i+1}$ if $s_i =$ "<";
  • $p_i \gt p_{i+1}$ if $s_i =$ ">".
翻译中…

As the result can be very large, you should print it modulo $998\,244\,353$.

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Input

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The first line contains two integers $n$ and $m$ ($2 \le n \le 2 \cdot 10^5$, $1 \leq m \leq \min(100, n-1)$).

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The second line contains a string $s$ of length $m$, consisting of characters "<" and ">".

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Output

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Print a single integer: the number of good permutations satisfying the constraints described in the statement, modulo $998\,244\,353$.

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Note

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In the first test, there are $5$ good permutations satisfying the constraints given by the string $s$: $[4, 3, 2, 1, 5]$, $[5, 3, 2, 1, 4]$, $[5, 4, 2, 1, 3]$, $[5, 4, 3, 1, 2]$, $[5, 4, 3, 2, 1]$. Each of them

翻译中…
  • is good;
  • satisfies $p_1 \gt p_2$;
  • satisfies $p_2 \gt p_3$;
  • satisfies $p_3 \gt p_4$.
翻译中…

In the second test, there are $60$ permutations such that $p_1 \lt p_2$. Only $56$ of them are good: the permutations $[1, 4, 3, 5, 2]$, $[1, 5, 3, 4, 2]$, $[2, 4, 3, 5, 1]$, $[2, 5, 3, 4, 1]$ are not good because the required condition doesn't hold for $(l, r)$ = $(1, 5)$. For example, for the permutation $[2, 4, 3, 5, 1]$,

翻译中…
  • the first minimum and the second minimum are $p_5$ and $p_1$, respectively (so they are $\{p_l, p_r\}$ up to reordering);
  • the third minimum is $p_3$ (neither $p_{l+1}$ nor $p_{r-1}$).
翻译中…

In the third test, there are $23$ good permutations satisfying the constraints given by the string $s$: $[1, 2, 4, 3, 6, 5]$, $[1, 2, 5, 3, 6, 4]$, $[1, 2, 6, 3, 5, 4]$, $[1, 3, 4, 2, 6, 5]$, $[1, 3, 5, 2, 6, 4]$, $[1, 3, 6, 2, 5, 4]$, $[1, 4, 5, 2, 6, 3]$, $[1, 4, 6, 2, 5, 3]$, $[1, 5, 6, 2, 4, 3]$, $[2, 3, 4, 1, 6, 5]$, $[2, 3, 5, 1, 6, 4]$, $[2, 3, 6, 1, 5, 4]$, $[2, 4, 5, 1, 6, 3]$, $[2, 4, 6, 1, 5, 3]$, $[2, 5, 6, 1, 4, 3]$, $[3, 4, 5, 1, 6, 2]$, $[3, 4, 5, 2, 6, 1]$, $[3, 4, 6, 1, 5, 2]$, $[3, 4, 6, 2, 5, 1]$, $[3, 5, 6, 1, 4, 2]$, $[3, 5, 6, 2, 4, 1]$, $[4, 5, 6, 1, 3, 2]$, $[4, 5, 6, 2, 3, 1]$.

翻译中…

样例

Input #1
5 3
>>>
Output #1
5
InputCopy OutputCopy
Input #2
5 1
<
Output #2
56
InputCopy OutputCopy
Input #3
6 5
<<><>
Output #3
23
InputCopy OutputCopy
Input #4
10 5
><<><
Output #4
83154
InputCopy OutputCopy
Input #5
1008 20
<><<>>><<<<<>>>>>>>>
Output #5
284142857
InputCopy OutputCopy
题目信息
题号
1654H
时间限制
8 s
内存限制
1024 MB
标签
组合数学
构造
分治
动态规划
FFT
数学

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。