PermutationCounts | Topcoder Archive

Problem Statement

You are given an int N and a int[] pos. We are interested in some permutations of the set {1,2,...,N}. A permutation p is called good if the following condition is satisfied: for each valid k, we have p(k) < p(k+1) if and only if k is an element of pos.

Return the number of good permutations, modulo 1,000,000,007.

Definition

Class:: PermutationCounts
Method:: countPermutations
Parameters:: int, int[]
Returns:: int
Method signature:: int countPermutations(int N, int[] pos)
(be sure your method is public)

Constraints

N will be between 1 and 1,000,000, inclusive.
pos will contain between 0 and min(N-1, 2500) elements, inclusive.
Elements of pos will be distinct.
Each element of pos will be between 1 and N-1, inclusive.

Examples

```
5
```
```
{3}
```
```
Returns: 9
```
Given that pos = {3}, we are looking for permutations where p(1) > p(2), p(2) > p(3), p(3) < p(4), and p(4) > p(5). Thus, the good permutations are the following ones: {3,2,1,5,4} {4,2,1,5,3} {4,3,1,5,2} {4,3,2,5,1} {5,2,1,4,3} {5,3,1,4,2} {5,3,2,4,1} {5,4,1,3,2} {5,4,2,3,1}

{12,11,10,9,8,7,6,5,4,3,2,1}

Returns: 1

```
13
```
```
{}
```
```
Returns: 1
```
```
9
```
```
{2,4,5}
```
```
Returns: 1421
```

{31,41,59,26,53,58,9,79,32,3,8,46}

Returns: 82650786

{295,311,98,345,420,547,646,734,380,325,608,783,141,65,305,437,769,252,44,
872,123,6,50,507,450,426,343,740,69,695,101,607,597,535,342,307,329,837,803,
237,459,444,498,15,712,134,473,14,715,223,787,192,710,750,193,293,242,652,
212,580,545,488,506,533,774,460,285,534,350,210,559,805,686,67,159,541,706,
514,657,801,373,754,310,800,589,736,863,675,254,283,604,27,628,103,81,235,
677,461,609,30,581,75,756,688,262,563,679,21,217,515,836,868,13,728,717,
309,267,767,259,414,332,744,129,859,4,179,632,415,278,812,79,77,784,573,433,
865,407,121,477,567,790,127,593,57,674,114,239,599,552,738}

Returns: 169176190

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