CodeCraft-20 (Div. 2)
F
Battalion Strength
Codeforces
2800
原题
← E→
翻译中…

There are $n$ officers in the Army of Byteland. Each officer has some power associated with him. The power of the $i$-th officer is denoted by $p_{i}$. As the war is fast approaching, the General would like to know the strength of the army.

翻译中…

The strength of an army is calculated in a strange way in Byteland. The General selects a random subset of officers from these $n$ officers and calls this subset a battalion.(All $2^n$ subsets of the $n$ officers can be chosen equally likely, including empty subset and the subset of all officers).

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The strength of a battalion is calculated in the following way:

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Let the powers of the chosen officers be $a_{1},a_{2},\ldots,a_{k}$, where $a_1 \le a_2 \le \dots \le a_k$. The strength of this battalion is equal to $a_1a_2 + a_2a_3 + \dots + a_{k-1}a_k$. (If the size of Battalion is $\leq 1$, then the strength of this battalion is $0$).

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The strength of the army is equal to the expected value of the strength of the battalion.

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As the war is really long, the powers of officers may change. Precisely, there will be $q$ changes. Each one of the form $i$ $x$ indicating that $p_{i}$ is changed to $x$.

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You need to find the strength of the army initially and after each of these $q$ updates.

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Note that the changes are permanent.

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The strength should be found by modulo $10^{9}+7$. Formally, let $M=10^{9}+7$. It can be shown that the answer can be expressed as an irreducible fraction $p/q$, where $p$ and $q$ are integers and $q\not\equiv 0 \bmod M$). Output the integer equal to $p\cdot q^{-1} \bmod M$. In other words, output such an integer $x$ that $0 \leq x \lt M$ and $x ⋅ q \equiv p \bmod M$).

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Input

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The first line of the input contains a single integer $n$ ($1 \leq n \leq 3⋅10^{5}$)  — the number of officers in Byteland's Army.

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The second line contains $n$ integers $p_{1},p_{2},\ldots,p_{n}$ ($1 \leq p_{i} \leq 10^{9}$).

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The third line contains a single integer $q$ ($1 \leq q \leq 3⋅10^{5}$)  — the number of updates.

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Each of the next $q$ lines contains two integers $i$ and $x$ ($1 \leq i \leq n$, $ 1 \leq x \leq 10^{9}$), indicating that $p_{i}$ is updated to $x$ .

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Output

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In the first line output the initial strength of the army.

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In $i$-th of the next $q$ lines, output the strength of the army after $i$-th update.

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Note

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In first testcase, initially, there are four possible battalions

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  • {} Strength = $0$
  • {$1$} Strength = $0$
  • {$2$} Strength = $0$
  • {$1,2$} Strength = $2$
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So strength of army is $\frac{0+0+0+2}{4}$ = $\frac{1}{2}$

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After changing $p_{1}$ to $2$, strength of battallion {$1,2$} changes to $4$, so strength of army becomes $1$.

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After changing $p_{2}$ to $1$, strength of battalion {$1,2$} again becomes $2$, so strength of army becomes $\frac{1}{2}$.

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样例

Input #1
2
1 2
2
1 2
2 1
Output #1
500000004
1
500000004
InputCopy OutputCopy
Input #2
4
1 2 3 4
4
1 5
2 5
3 5
4 5
Output #2
625000011
13
62500020
375000027
62500027
InputCopy OutputCopy
题目信息
题号
1316F
时间限制
5 s
内存限制
256 MB
标签
数据结构
分治
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。