No.1520 Zigzag Sum
タグ : / 解いたユーザー数 81
作問者 :
aspi
penguinman
👑
Nachia
Problem Statement
There is a grid with $H$ rows and $W$ columns. The cell in the $i$-th row from the top and the $j$-th column from the left is denoted by $(i,j)$.
Natsubi moves on this grid from $(1,1)$ to $(H,W)$. When he is at $(i,j)$, he can move to $(i+1,j)$ or $(i,j+1)$. He cannot move outside the grid.
For a given path, its zigzag degree is defined as follows.
- The number of cells on the path at which the path turns
Here, "the path turns" at a cell $(i, j)$ means that at least one of the following conditions holds.
- $(i-1,j),\ (i,j),\ (i,j+1)$ all exist, and the path passes through all of them.
- $(i,j-1),\ (i,j),\ (i+1,j)$ all exist, and the path passes through all of them.
Given $H,W$, find the zigzag degree of each of the $\displaystyle \binom{H+W-2}{H-1}$ possible paths, and report the sum of these values.
Since the answer can be very large, output it modulo $10^9+7$ (a prime).
Solve $T$ test cases.
Input
The input is given from standard input. The $1$st line of the input is as follows.
$T$
Then, $T$ test cases are given on the following $T$ lines. Each of them is in the following format.
$H\ W$
- All input values are integers.
- $1 \leq T \leq 10^5$
- $1 \leq H,W \leq 2 \times 10^5$
Output
For each test case, output the answer. Print a newline after each test case.
Sample
Sample 1
Input
4 2 3 1 10 1 1 200000 200000
Output
4 0 0 316943316
- For the $1$st case: the possible paths are the $3$ shown in the figure below. Their zigzag degrees are $1,2,1$ respectively, so the answer is $4$.
- For the $2$nd case: there is no way to turn in the first place.
- For the $3$rd case: he does not even move, but the answer is still $0$.
- For the $4$th case: do not forget to output the answer modulo $10^9+7$.
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