Codeforces Round 695 (Div. 2)
D
Sum of Paths
Codeforces
2200
原题
← CE →
翻译中…

There are $n$ cells, numbered $1,2,\dots, n$ from left to right. You have to place a robot at any cell initially. The robot must make exactly $k$ moves.

翻译中…

In one move, the robot must move one cell to the left or right, provided that it doesn't move out of bounds. In other words, if the robot was in the cell $i$, it must move to either the cell $i-1$ or the cell $i+1$, as long as it lies between $1$ and $n$ (endpoints inclusive). The cells, in the order they are visited (including the cell the robot is placed), together make a good path.

翻译中…

Each cell $i$ has a value $a_i$ associated with it. Let $c_0, c_1, \dots, c_k$ be the sequence of cells in a good path in the order they are visited ($c_0$ is the cell robot is initially placed, $c_1$ is the cell where the robot is after its first move, and so on; more formally, $c_i$ is the cell that the robot is at after $i$ moves). Then the value of the path is calculated as $a_{c_0} + a_{c_1} + \dots + a_{c_k}$.

翻译中…

Your task is to calculate the sum of values over all possible good paths. Since this number can be very large, output it modulo $10^9 + 7$. Two good paths are considered different if the starting cell differs or there exists an integer $i \in [1, k]$ such that the current cell of the robot after exactly $i$ moves is different in those paths.

翻译中…

You must process $q$ updates to $a$ and print the updated sum each time. Each update changes the value of exactly one cell. See the input format and the sample input-output for more details.

翻译中…

Input

翻译中…

The first line of the input contains three space-separated integers $n$, $k$ and $q$ ($2 \le n \le 5000$; $1 \le k \le 5000$; $1 \le q \le 2 \cdot 10^5$).

翻译中…

The second line of the input contains $n$ integers $a_1, a_2, \dots, a_n$ ($1 \le a_i \le 10^9$).

翻译中…

$q$ lines follow. Each line contains two space-separated integers $i$ and $x$ ($1 \le i \le n$; $1 \le x \le 10^9$) indicating that you must change the value of $a_i$ to $x$.

翻译中…

Output

翻译中…

Print $q$ integers. The $i$-th integer should be the sum of values over all good paths after the first $i$ updates are performed. Since the answers may be large, print them modulo $10^9 + 7$.

翻译中…

Note

翻译中…

In the first example, the good paths are $(1, 2), (2, 1), (2, 3), (3, 2), (3, 4), (4, 3), (4, 5), (5, 4)$.

翻译中…

Initially the values of $a$ are $[3, 5, 1, 4, 2]$. After the first update, they become $[9, 5, 1, 4, 2]$. After the second update, they become $[9, 4, 1, 4, 2]$, and so on.

翻译中…

样例

Input #1
5 1 5
3 5 1 4 2
1 9
2 4
3 6
4 6
5 2
Output #1
62
58
78
86
86
InputCopy OutputCopy
Input #2
5 2 5
3 5 1 4 2
1 9
2 4
3 6
4 6
5 2
Output #2
157
147
207
227
227
InputCopy OutputCopy
Input #3
4 40 6
92 21 82 46
3 56
1 72
4 28
1 97
2 49
2 88
Output #3
239185261
666314041
50729936
516818968
766409450
756910476
InputCopy OutputCopy
题目信息
题号
1467D
时间限制
3 s
内存限制
1024 MB
标签
组合数学
动态规划
数学

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