ABC122-D We Like AGC

問題へのリンク

C#のソース

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

class Program
{
    static string InputPattern = "InputX";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("3");
            //61
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("4");
            //230
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("100");
            //388130742
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    const long Hou = 1000000007;

    static void Main()
    {
        List<string> InputList = GetInputList();
        long N = long.Parse(InputList[0]);

        // 場合の数[最後の4文字]なDP表
        var PrevDP = new Dictionary<string, long>();
        PrevDP[""] = 1;

        for (long I = 1; I <= N; I++) {
            var CurrDP = new Dictionary<string, long>();

            foreach (var EachPair in PrevDP) {
                foreach (char EachChar in "ACGT") {
                    string NewKey = EachPair.Key + EachChar;

                    if (NewKey.EndsWith("AGC")) continue;
                    if (NewKey.EndsWith("GAC")) continue;
                    if (NewKey.EndsWith("ACG")) continue;
                    if (Regex.IsMatch(NewKey, "A.GC")) continue;
                    if (Regex.IsMatch(NewKey, "AG.C")) continue;

                    // 最後の4文字のみが必要な情報
                    if (NewKey.Length > 4) {
                        int StrUB = NewKey.Length - 1;
                        NewKey = NewKey.Substring(StrUB - 3);
                    }

                    if (CurrDP.ContainsKey(NewKey) == false) {
                        CurrDP[NewKey] = 0;
                    }
                    CurrDP[NewKey] += EachPair.Value;
                    CurrDP[NewKey] %= Hou;
                }
            }
            PrevDP = CurrDP;
        }

        long Answer = 0;
        foreach (long EachLong in PrevDP.Values) {
            Answer += EachLong;
            Answer %= Hou;
        }
        Console.WriteLine(Answer);
    }
}

解説

最後の4文字を状態に持つ動的計画法で解いてます。