Problem H. RMQ Similar Sequence
时间限制 2000 ms
内存限制 249.5458984375 MB
Chiaki has a sequence $A=\{a_1,a_2,\dots,a_n\}$. Let $\mathbf{RMQ}(A, l, r)$ be the minimum $i$ ($l \le i \le r$) such that $a_i$ is the maximum value in $a_l, a_{l+1}, \dots, a_{r}$.
Two sequences $A$ and $B$ are called \textit{RMQ Similar}, if they have the same length $n$ and for every $1 \le l \le r \le n$, $\mathbf{RMQ}(A, l, r) = \mathbf{RMQ}(B, l, r)$.
For a given the sequence $A=\{a_1,a_2,\dots,a_n\}$, define the weight of a sequence $B=\{b_1,b_2,\dots,b_n\}$ be $\sum\limits_{i=1}^{n} b_i$ (i.e. the sum of all elements in $B$) if sequence $B$ and sequence $A$ are RMQ Similar, or $0$ otherwise. If each element of $B$ is a real number chosen independently and uniformly at random between $0$ and $1$, find the expected weight of $B$.
输入数据
输出数据
For each test case, output the answer as a value of a rational number modulo $10^9 + 7$.
Formally, it is guaranteed that under given constraints the probability is always a rational number $\frac{p}{q}$ ($p$ and $q$ are integer and coprime, $q$ is positive), such that $q$ is not divisible by $10^9 + 7$. Output such integer a between $0$ and $10^9 + 6$ that $p - aq$ is divisible by $10^9 + 7$.
样例输入
复制
3
3
1 2 3
3
1 2 1
5
1 2 3 2 1
样例输出
复制
250000002
500000004
125000001
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