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USACO 2024 December Contest, Platinum

Problem 3. Maximize Minimum Difference


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**Note: The time limit for this problem is 4s, twice the default.**

Moo! You are given an integer N (2N2000). Consider all permutations p=[p0,p1,,pN1] of [0,1,2,N1]. Let f(p)=min denote the minimum absolute difference between any two consecutive elements of p. and let S denote the set of all p that achieve the maximum possible value of f(p).

You are additionally given K (0\le K\le N) constraints of the form p_i=j (0\le i,j<N). Count the number of permutations in S satisfying all constraints, modulo 10^9+7.

INPUT FORMAT (input arrives from the terminal / stdin):

The first line contains T (1\le TN\le 2\cdot 10^4) and N, meaning that you will need to solve T independent test cases, each specified by a different set of constraints.

Each test case starts with K, followed by K lines each containing i and j. It is guaranteed that

  • The same i does not appear more than once within the same test case.
  • The same j does not appear more than once within the same test case.

OUTPUT FORMAT (print output to the terminal / stdout):

For each test case, the answer modulo 10^9+7 on a separate line.

SAMPLE INPUT:

3 4
0
1
1 1
2
0 2
2 3

SAMPLE OUTPUT:

2
0
1

The maximum possible value of f(p) is 2, and S=\{[2,0,3,1], [1,3,0,2]\}.

SAMPLE INPUT:

9 11
2
0 5
6 9
3
0 5
6 9
1 0
4
0 5
6 9
1 0
4 7
5
0 5
6 9
1 0
4 7
2 6
6
0 5
6 9
1 0
4 7
2 6
9 3
7
0 5
6 9
1 0
4 7
2 6
9 3
5 2
8
0 5
6 9
1 0
4 7
2 6
9 3
5 2
7 4
9
0 5
6 9
1 0
4 7
2 6
9 3
5 2
7 4
3 1
10
0 5
6 9
1 0
4 7
2 6
9 3
5 2
7 4
3 1
8 10

SAMPLE OUTPUT:

6
6
1
1
1
1
1
1
1

p=[5, 0, 6, 1, 7, 2, 9, 4, 10, 3, 8] should be counted for all test cases.

SAMPLE INPUT:

10 11
0
1
3 8
2
3 8
5 7
3
3 8
5 7
4 2
4
3 8
5 7
4 2
10 6
5
3 8
5 7
4 2
10 6
8 10
6
3 8
5 7
4 2
10 6
8 10
1 9
7
3 8
5 7
4 2
10 6
8 10
1 9
7 5
8
3 8
5 7
4 2
10 6
8 10
1 9
7 5
2 3
9
3 8
5 7
4 2
10 6
8 10
1 9
7 5
2 3
6 0

SAMPLE OUTPUT:

160
20
8
7
2
1
1
1
1
1

p=[4, 9, 3, 8, 2, 7, 0, 5, 10, 1, 6] should be counted for all test cases.

SAMPLE INPUT:

5 987
3
654 321
543 210
432 106
2
654 321
543 210
1
654 321
1
0 493
0

SAMPLE OUTPUT:

0
538184948
693625420
932738155
251798971

Make sure to output the answer modulo 10^9+7.

SCORING:

  • Input 5: N=15
  • Input 6: N=2000
  • Inputs 7-9: For all test cases, the constraint p_0=\lfloor N/2\rfloor appears.
  • Inputs 10-13: For all test cases, there exists some constraint p_i = j with j equal to \lfloor N/2\rfloor.
  • Inputs 14-20: No additional constraints.

Problem credits: Benjamin Qi

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