Problem:
Score : 400 points

Problem Statement

You are given an integer N. Find the number of strings of length N that satisfy the following conditions, modulo 10^9+7:

The string does not contain characters other than A, C, G and T.

The string does not contain AGC as a substring.

The condition above cannot be violated by swapping two adjacent characters once.

Notes

A substring of a string T is a string obtained by removing zero or more characters from the beginning and the end of T.

For example, the substrings of ATCODER include TCO, AT, CODER, ATCODER and (the empty string), but not AC.

Constraints

3 \leq N \leq 100

Input

Input is given from Standard Input in the following format:

N

Output

Print the number of strings of length N that satisfy the following conditions, modulo 10^9+7.

Sample Input 1

3

Sample Output 1

61

There are 4^3 = 64 strings of length 3 that do not contain characters other than A, C, G and T. Among them, only AGC, ACG and GAC violate the condition, so the answer is 64 - 3 = 61.

Sample Input 2

4

Sample Output 2

230

Sample Input 3

100

Sample Output 3

388130742

Be sure to print the number of strings modulo 10^9+7.

Solution in Scala (2.11.7) (judge verdict: Accepted)

import java.util.Scanner

import scala.collection.mutable


object Main {

  case class ResidueRing(private val input: Long) {
    val representative: Long = ((input % ResidueRing.mod) + ResidueRing.mod) % ResidueRing.mod

    def +(that: ResidueRing): ResidueRing = ResidueRing(this.representative + that.representative)

    def -(that: ResidueRing): ResidueRing = ResidueRing(this.representative - that.representative)

    def *(that: ResidueRing): ResidueRing = ResidueRing(this.representative * that.representative)

    def unary_- : ResidueRing = ResidueRing(-this.representative)

  }

  object ResidueRing {
    val mod: Long = (1e9 + 7).toLong
    val _0: ResidueRing = ResidueRing(0)
    val _1: ResidueRing = ResidueRing(1)
  }


  def read() = {
    val sc = new Scanner(System.in)
    val n = sc.nextInt()
    n
  }


  def solve(n: Int): Long = {
    // 過去3文字分記憶しておけばいい
    val alphabets = "ATGC"
    val stateList: IndexedSeq[String] =
      for (c1 <- alphabets;
           c2 <- alphabets;
           c3 <- alphabets) yield IndexedSeq(c1, c2, c3).mkString


    def transition(state: String, alphabet: Char): Option[String] = {
      (state(0), state(1), state(2), alphabet) match {
        case (_, 'A', 'G', 'C')
             | (_, 'G', 'A', 'C')
             | (_, 'A', 'C', 'G')
             | ('A', _, 'G', 'C')
             | ('A', 'G', _, 'C') => None
        case _ => Some(IndexedSeq(state(1), state(2), alphabet).mkString)
      }
    }

    val mapAll0: mutable.Map[String, ResidueRing] = mutable.Map.empty
    for (state <- stateList) {
      mapAll0.+=((state, ResidueRing._0))
    }
    val dp: Array[mutable.Map[String, ResidueRing]] = Array.fill(n + 1)(mapAll0.clone)

    dp(0)("TTT") += ResidueRing._1
    for (i <- 0 until n) {
      for (state <- stateList) {
        for (alphabet <- alphabets) {
          val next = transition(state, alphabet)
          next match {
            case Some(nextState) => dp(i + 1)(nextState) += dp(i)(state)
            case None =>
          }

        }
      }
    }
    val result = dp.last.values.foldLeft(ResidueRing._0)(_ + _)
    result.representative
  }


  def main(args: Array[String]): Unit = {
    val n = read()
    println(solve(n))
  }
}