ABC276-E Round Trip

問題へのリンク

C#のソース

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static string InputPattern = "InputX";

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

        if (InputPattern == "Input1") {
            WillReturn.Add("4 4");
            WillReturn.Add("....");
            WillReturn.Add("#.#.");
            WillReturn.Add(".S..");
            WillReturn.Add(".##.");
            //Yes
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("2 2");
            WillReturn.Add("S.");
            WillReturn.Add(".#");
            //No
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("5 7");
            WillReturn.Add(".#...#.");
            WillReturn.Add("..#.#..");
            WillReturn.Add("...S...");
            WillReturn.Add("..#.#..");
            WillReturn.Add(".#...#.");
            //No
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static char[,] mBanArr;
    static int UB_X;
    static int UB_Y;

    static int StaX;
    static int StaY;

    static void Main()
    {
        List<string> InputList = GetInputList();
        mBanArr = CreateBanArr(InputList.Skip(1));
        UB_X = mBanArr.GetUpperBound(0);
        UB_Y = mBanArr.GetUpperBound(1);

        for (int X = 0; X <= UB_X; X++) {
            for (int Y = 0; Y <= UB_Y; Y++) {
                if (mBanArr[X, Y] == 'S') {
                    StaX = X;
                    StaY = Y;
                }
            }
        }

        if (ExecBFS(0, -1)) {
            Console.WriteLine("Yes"); return;
        }
        if (ExecBFS(0, +1)) {
            Console.WriteLine("Yes"); return;
        }
        if (ExecBFS(-1, 0)) {
            Console.WriteLine("Yes"); return;
        }
        if (ExecBFS(+1, 0)) {
            Console.WriteLine("Yes"); return;
        }
        Console.WriteLine("No");
    }

    static bool ExecBFS(int pVectX, int pVectY)
    {
        var Que = new Queue<JyoutaiDef>();
        JyoutaiDef WillEnqueue;
        WillEnqueue.Curr_X = StaX;
        WillEnqueue.Curr_Y = StaY;
        WillEnqueue.Level = 0;
        Que.Enqueue(WillEnqueue);

        var VisitedSet = new HashSet<long>();
        while (Que.Count > 0) {
            JyoutaiDef Dequeued = Que.Dequeue();

            // クリア判定
            if (Dequeued.Curr_X == StaX && Dequeued.Curr_Y == StaY) {
                if (Dequeued.Level == 2) continue;
                if (Dequeued.Level >= 4) {
                    return true;
                }
            }

            Action<int, int> EnqueueAct = (pNewX, pNewY) =>
            {
                if (pNewX < 0 || UB_X < pNewX) return;
                if (pNewY < 0 || UB_Y < pNewY) return;

                if (mBanArr[pNewX, pNewY] == '#') return;

                long Hash = GetHash(pNewX, pNewY);
                bool CanEnqueue = false;

                // ゴールは再訪問OK
                if (pNewX == StaX && pNewY == StaY) {
                    CanEnqueue = true;
                }
                else if (VisitedSet.Add(Hash)) {
                    CanEnqueue = true;
                }

                if (CanEnqueue) {
                    WillEnqueue.Curr_X = pNewX;
                    WillEnqueue.Curr_Y = pNewY;
                    WillEnqueue.Level = Dequeued.Level + 1;
                    Que.Enqueue(WillEnqueue);
                }
            };

            if (Dequeued.Level == 0) {
                EnqueueAct(Dequeued.Curr_X + pVectX, Dequeued.Curr_Y + pVectY);
            }
            else {
                EnqueueAct(Dequeued.Curr_X, Dequeued.Curr_Y - 1);
                EnqueueAct(Dequeued.Curr_X, Dequeued.Curr_Y + 1);
                EnqueueAct(Dequeued.Curr_X - 1, Dequeued.Curr_Y);
                EnqueueAct(Dequeued.Curr_X + 1, Dequeued.Curr_Y);
            }
        }
        return false;
    }

    struct JyoutaiDef
    {
        internal int Curr_X;
        internal int Curr_Y;
        internal int Level;
    }

    static long GetHash(int pX, int pY)
    {
        return (long)pX * 10000000 + pY;
    }

    ////////////////////////////////////////////////////////////////
    // IEnumerable<string>をcharの2次元配列に設定する
    ////////////////////////////////////////////////////////////////
    static char[,] CreateBanArr(IEnumerable<string> pStrEnum)
    {
        var StrList = pStrEnum.ToList();
        if (StrList.Count == 0) {
            return new char[0, 0];
        }
        int UB_X = StrList[0].Length - 1;
        int UB_Y = StrList.Count - 1;

        char[,] WillReturn = new char[UB_X + 1, UB_Y + 1];

        for (int Y = 0; Y <= UB_Y; Y++) {
            for (int X = 0; X <= UB_X; X++) {
                WillReturn[X, Y] = StrList[Y][X];
            }
        }
        return WillReturn;
    }
}

解説

最初の移動は、4近傍のいずれかのみ可能、
2手でスタートに戻るのは不可としてBFSしてます。

最初の移動が4通りあるので、
4回BFSを行い、どれかのBFSでスタートに戻れるかを判定してます。