AlienAndSetDiv1 | Topcoder Archive

Problem Statement

Alien Fred wants to destroy the Earth. But before he does that, he wants to solve the following problem.

He has the set {1, 2, 3, ..., 2N}. He wants to split this set into two new sets A and B. The following conditions must all be satisfied:

Each element of the original set must belong to exactly one of the sets A and B.
The two new sets must have the same size. (I.e., each of them must contain exactly N numbers.)
For each i from 1 to N, inclusive: Let A[i] be the i-th smallest element of A, and let B[i] be the i-th smallest element of B. The difference |A[i] - B[i]| must be greater than or equal to K.

You are given the two ints N and K. Let X be the total number of ways in which Fred can choose the sets A and B. Return the value (X modulo 1,000,000,007).

Definition

Class:: AlienAndSetDiv1
Method:: getNumber
Parameters:: int, int
Returns:: int
Method signature:: int getNumber(int N, int K)
(be sure your method is public)

Constraints

N will be between 1 and 50, inclusive.
K will be between 1 and 10, inclusive.

Examples

```
2
```
```
2
```
```
Returns: 2
```
The initial set is {1, 2, 3, 4}. The following 6 pairs of subsets are possible in this case: A={1, 2} and B={3, 4} A={1, 3} and B={2, 4} A={1, 4} and B={2, 3} A={2, 3} and B={1, 4} A={2, 4} and B={1, 3} A={3, 4} and B={1, 2} The first option and the last option are both valid. The other 4 options are invalid. Note that order of the two sets matters: the option A={1,2} and B={3,4} differs from the option A={3,4} and B={1,2}.
```
3
```
```
1
```
```
Returns: 20
```
```
4
```
```
2
```
```
Returns: 14
```
```
10
```
```
7
```
```
Returns: 40
```

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