Deltix Round, Spring 2021 (open for everyone, rated, Div. 1 + Div. 2)
E
Crypto Lights
Codeforces
2600
原题
← DF →
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image

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To monitor cryptocurrency exchange rates trader William invented a wonderful device consisting of $n$ lights arranged in a row. The device functions in the following way:

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Initially, all lights on William's device are turned off. At the beginning of a new iteration the device randomly, with a uniform distribution, picks a light that is turned off and turns it on, telling William which cryptocurrency he should invest in. After this iteration if any $k$ consecutive lights contain more than one turned on light, then the device finishes working.

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William doesn't like uncertainty, so he wants you to calculate the expected value of the number of lights that are turned on in the device after it finishes working.

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Input

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Each test contains multiple test cases. The first line contains the number of test cases $t$ ($1 \le t \le 10$). Description of the test cases follows.

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The only line for each test case contains two integers $n$ and $k$ ($2 \le k \le n \le 10^5$), which are the total number of lights and the length of subsegment of lights that are being checked, respectively.

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Output

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For each test case print the answer, modulo $10^9+7$.

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Formally, let $M = 10^9+7$. It can be shown that the answer can be expressed as an irreducible fraction $\frac{p}{q}$, where $p$ and $q$ are integers and $q \not \equiv 0 \pmod{M}$. Output the integer equal to $p \cdot q^{-1} \bmod M$. In other words, output such an integer $x$ that $0 \le x \lt M$ and $x \cdot q \equiv p \pmod{M}$.

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Note

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Explanation of the first sample test case:

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Let's write out all possible sequences of light toggles, which will make the device complete its operation:

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  1. $(1, 2)$ — $2$ lights are turned on
  2. $(1, 3, 2)$ — $3$ lights are turned on
  3. $(2, 1)$ — $2$ lights are turned on
  4. $(2, 3)$ — $2$ lights are turned on
  5. $(3, 2)$ — $2$ lights are turned on
  6. $(3, 1, 2)$ — $3$ lights are turned on
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Then the final expected value will be equal to $\frac{2}{6}$ + $\frac{3}{6}$ + $\frac{2}{6}$ + $\frac{2}{6}$ + $\frac{2}{6}$ + $\frac{3}{6}$ = $\frac{14}{6}$ = $\frac{7}{3}$.

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Then the required output will be $333333338$, since $333333338 \cdot 3 \equiv 7 \pmod{10^9+7}$.

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样例

Input #1
3
3 2
15 2
40 15
Output #1
333333338
141946947
329622137
InputCopy OutputCopy
题目信息
题号
1523E
时间限制
3 s
内存限制
256 MB
标签
组合数学
动态规划
数学
概率论

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。