Codeforces Round 728 (Div. 1) |
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Finished |
This is the easy version of the problem. The only difference is that in this version $$$q = 1$$$. You can make hacks only if both versions of the problem are solved.
There is a process that takes place on arrays $$$a$$$ and $$$b$$$ of length $$$n$$$ and length $$$n-1$$$ respectively.
The process is an infinite sequence of operations. Each operation is as follows:
It can be proven that array $$$a$$$ converges, i. e. for each $$$i$$$ there exists a limit $$$a_i$$$ converges to. Let function $$$F(a, b)$$$ return the value $$$a_1$$$ converges to after a process on $$$a$$$ and $$$b$$$.
You are given array $$$b$$$, but not array $$$a$$$. However, you are given a third array $$$c$$$. Array $$$a$$$ is good if it contains only integers and satisfies $$$0 \leq a_i \leq c_i$$$ for $$$1 \leq i \leq n$$$.
Your task is to count the number of good arrays $$$a$$$ where $$$F(a, b) \geq x$$$ for $$$q$$$ values of $$$x$$$. Since the number of arrays can be very large, print it modulo $$$10^9+7$$$.
The first line contains a single integer $$$n$$$ ($$$2 \le n \le 100$$$).
The second line contains $$$n$$$ integers $$$c_1, c_2 \ldots, c_n$$$ ($$$0 \le c_i \le 100$$$).
The third line contains $$$n-1$$$ integers $$$b_1, b_2, \ldots, b_{n-1}$$$ ($$$0 \le b_i \le 100$$$).
The fourth line contains a single integer $$$q$$$ ($$$q=1$$$).
The fifth line contains $$$q$$$ space separated integers $$$x_1, x_2, \ldots, x_q$$$ ($$$-10^5 \le x_i \le 10^5$$$).
Output $$$q$$$ integers, where the $$$i$$$-th integer is the answer to the $$$i$$$-th query, i. e. the number of good arrays $$$a$$$ where $$$F(a, b) \geq x_i$$$ modulo $$$10^9+7$$$.
3 2 3 4 2 1 1 -1
56
The following explanation assumes $$$b = [2, 1]$$$ and $$$c=[2, 3, 4]$$$ (as in the sample).
Examples of arrays $$$a$$$ that are not good:
One possible good array $$$a$$$ is $$$[0, 2, 4]$$$. We can show that no operation has any effect on this array, so $$$F(a, b) = a_1 = 0$$$.
Another possible good array $$$a$$$ is $$$[0, 1, 4]$$$. In a single operation with $$$i = 1$$$, we set $$$a_1 = \min(\frac{0+1-2}{2}, 0)$$$ and $$$a_2 = \max(\frac{0+1+2}{2}, 1)$$$. So, after a single operation with $$$i = 1$$$, $$$a$$$ becomes equal to $$$[-\frac{1}{2}, \frac{3}{2}, 4]$$$. We can show that no operation has any effect on this array, so $$$F(a, b) = -\frac{1}{2}$$$.
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