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CodeChef: Chef and Land

問題

https://www.codechef.com/problems/CHEFIHG

N x M の 2 次元平面のマップが与えられる。マップには道 '.' と壁 '*' と首都 'C' がある。ロボットがあり、ロボットに U, R, L, D のコマンドを与えることで上下左右に動かすことが出来る。

全ての道にロボットがいて、全てのロボットに同じ命令の列を与えるとき、全てのロボットが命令の実行途中で首都を通るような命令を出力せよ。

解法

まだ首都に行っていないロボットがいれば首都に向かわせる。最悪 400 台くらいのロボットがいて、最悪 400 マスくらい移動するので、100,000 命令以内で必ず全てのロボットが首都を通る。

コード

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.NoSuchElementException;

/*
                   _ooOoo_
                  o8888888o
                  88" . "88
                  (| -_- |)
                  O\  =  /O
               ____/`---'\____
             .'  \\|     |//  `.
            /  \\|||  :  |||//  \
           /  _||||| -:- |||||-  \
           |   | \\\  -  /// |   |
           | \_|  ''\---/''  |   |
           \  .-\__  `-`  ___/-. /
         ___`. .'  /--.--\  `. . __
      ."" '<  `.___\_<|>_/___.'  >'"".
     | | :  `- \`.;`\ _ /`;.`/ - ` : | |
     \  \ `-.   \_ __\ /__ _/   .-` /  /
======`-.____`-.___\_____/___.-`____.-'======
                   `=---='
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
            pass System Test!
*/

class CHEFIHG {
  private static class Task {
    final int[] R = new int[]{0, 0, 1, -1};
    final int[] C = new int[]{1, -1, 0, 0};
    final String dirs = "RLDU";
    int N, M;

    void solve(FastScanner in, PrintWriter out) {
      N = in.nextInt();
      M = in.nextInt();
      char[][] board = in.nextCharMap(N);

      int gr = -1, gc = -1;
      for (int i = 0; i < N; i++)
        for (int j = 0; j < M; j++)
          if (board[i][j] == 'C') {
            gr = i;
            gc = j;
          }

      boolean[][] now = new boolean[N][M];
      for (int i = 0; i < N; i++)
        for (int j = 0; j < M; j++)
          if (board[i][j] == '.') now[i][j] = true;

      char[][] bfsDir = new char[N][M];
      ArrayDeque<int[]> deque = new ArrayDeque<>();
      deque.add(new int[]{gr, gc});
      while (!deque.isEmpty()) {
        int[] p = deque.poll();
        int r = p[0];
        int c = p[1];
        for (int d = 0; d < 4; d++) {
          int nr = r + R[d];
          int nc = c + C[d];
          if (board[nr][nc] == '.' && bfsDir[nr][nc] == 0) {
            char dir;
            if (dirs.charAt(d) == 'D') dir = 'U';
            else if (dirs.charAt(d) == 'U') dir = 'D';
            else if (dirs.charAt(d) == 'R') dir = 'L';
            else dir = 'R';
            bfsDir[nr][nc] = dir;
            deque.add(new int[]{nr, nc});
          }
        }
      }

      StringBuilder ans = new StringBuilder();
      ArrayList<Integer> route = new ArrayList<>();
      while (true) {
        boolean[][] next = new boolean[N][M];
        route.clear();
        for (int i = 0; i < N; i++)
          for (int j = 0; j < M; j++) {
            if (!now[i][j]) continue;
            for (int r = i, c = j; board[r][c] != 'C'; ) {
              ans.append(bfsDir[r][c]);
              int d = dirs.indexOf(bfsDir[r][c]);
              r = r + R[d];
              c = c + C[d];
              route.add(d);
            }
          }

        for (int i = 0; i < N; i++) {
          for (int j = 0; j < M; j++) {
            if (!now[i][j]) continue;
            int r = i, c = j;
            for (int d : route) {
              int nr = r + R[d];
              int nc = c + C[d];
              if (board[nr][nc] != '*') {
                r = nr;
                c = nc;
                if (r == gr && c == gc) break;
              }
            }
            if (board[r][c] == '.') next[r][c] = true;
          }
        }

        if (route.isEmpty()) break;
        now = next;
      }
      out.println(ans.toString());
    }
  }

  /**
   * ここから下はテンプレートです。
   */
  public static void main(String[] args) {
    OutputStream outputStream = System.out;
    FastScanner in = new FastScanner();
    PrintWriter out = new PrintWriter(outputStream);
    Task solver = new Task();
    solver.solve(in, out);
    out.close();
  }
  private static class FastScanner {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int bufferLength = 0;

    private boolean hasNextByte() {
      if (ptr < bufferLength) {
        return true;
      } else {
        ptr = 0;
        try {
          bufferLength = in.read(buffer);
        } catch (IOException e) {
          e.printStackTrace();
        }
        if (bufferLength <= 0) {
          return false;
        }
      }
      return true;
    }

    private int readByte() {
      if (hasNextByte()) return buffer[ptr++];
      else return -1;
    }

    private static boolean isPrintableChar(int c) {
      return 33 <= c && c <= 126;
    }

    private void skipUnprintable() {
      while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
    }

    boolean hasNext() {
      skipUnprintable();
      return hasNextByte();
    }

    public String next() {
      if (!hasNext()) throw new NoSuchElementException();
      StringBuilder sb = new StringBuilder();
      int b = readByte();
      while (isPrintableChar(b)) {
        sb.appendCodePoint(b);
        b = readByte();
      }
      return sb.toString();
    }

    long nextLong() {
      if (!hasNext()) throw new NoSuchElementException();
      long n = 0;
      boolean minus = false;
      int b = readByte();
      if (b == '-') {
        minus = true;
        b = readByte();
      }
      if (b < '0' || '9' < b) {
        throw new NumberFormatException();
      }
      while (true) {
        if ('0' <= b && b <= '9') {
          n *= 10;
          n += b - '0';
        } else if (b == -1 || !isPrintableChar(b)) {
          return minus ? -n : n;
        } else {
          throw new NumberFormatException();
        }
        b = readByte();
      }
    }

    double nextDouble() {
      return Double.parseDouble(next());
    }

    double[] nextDoubleArray(int n) {
      double[] array = new double[n];
      for (int i = 0; i < n; i++) {
        array[i] = nextDouble();
      }
      return array;
    }

    double[][] nextDoubleMap(int n, int m) {
      double[][] map = new double[n][];
      for (int i = 0; i < n; i++) {
        map[i] = nextDoubleArray(m);
      }
      return map;
    }

    public int nextInt() {
      return (int) nextLong();
    }

    public int[] nextIntArray(int n) {
      int[] array = new int[n];
      for (int i = 0; i < n; i++) array[i] = nextInt();
      return array;
    }

    public long[] nextLongArray(int n) {
      long[] array = new long[n];
      for (int i = 0; i < n; i++) array[i] = nextLong();
      return array;
    }

    public String[] nextStringArray(int n) {
      String[] array = new String[n];
      for (int i = 0; i < n; i++) array[i] = next();
      return array;
    }

    public char[][] nextCharMap(int n) {
      char[][] array = new char[n][];
      for (int i = 0; i < n; i++) array[i] = next().toCharArray();
      return array;
    }
  }
}