Problem G. Chiaki Sequence Revisited
时间限制 1000 ms
内存限制 32 MB
Chiaki is interested in an infinite sequence $a_1, a_2, a_3, ...$, which is defined as follows:
$$a_n=\begin{cases}1 & n = 1,2 \\ a_{n - a_{n-1}} + a_{n-1 - a_{n-2}} & n \ge 3\end{cases}$$
Chiaki would like to know the sum of the first $n$ terms of the sequence, i.e. $\sum\limits_{i=1}^{n}a_i$. As this number may be very large, Chiaki is only interested in its remainder modulo ($10^9 + 7$).
输入数据
输出数据
For each test case, output an integer denoting the answer.
样例输入
复制
10
1
2
3
4
5
6
7
8
9
10
样例输出
复制
1
2
4
6
9
13
17
21
26
32
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