240625 불!

static final int WALL = -1;
static final int FIRE = -2;
static final int EMPTY = 0;
static final int VISITED = 1;

static final int MX = 1000;
static final int[] dx = { 0, 1, 0, -1 };
static final int[] dy = { 1, 0, -1, 0 };

static int r;
static int c;
static int[][] posStates = new int[MX][MX];
static Deque<Position> posQ = new ArrayDeque<>();

public static void main(String[] args) throws IOException {
	readMaze();
	int minTimeToEscape = getMinTimeToEscape();
	String result = minTimeToEscape == -1 ? "IMPOSSIBLE" : Integer.toString(minTimeToEscape);
	System.out.println(result);
}

static void readMaze() throws IOException {
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	StringTokenizer st = new StringTokenizer(br.readLine());
	r = Integer.parseInt(st.nextToken());
	c = Integer.parseInt(st.nextToken());
	for (int y = 0; y < r; ++y) {
		for (int x = 0; x < c; ++x) {
			char ch = (char) br.read();
			if (ch == '#')
				posStates[y][x] = WALL;
			else if (ch == '.')
				posStates[y][x] = EMPTY;
			else if (ch == 'F') {
				posStates[y][x] = FIRE;
				posQ.addFirst(new Position(x, y, FIRE));
			} else {
				posStates[y][x] = VISITED;
				posQ.addLast(new Position(x, y, VISITED));
			}
		}
		br.read();
	}
}

static int getMinTimeToEscape() {
	int escapeMinute = 0;
	do {
		escapeMinute++;
		int posQSize = posQ.size();
		while (posQSize-- > 0) {
			Position pos = posQ.poll();
			int x = pos.x;
			int y = pos.y;
			if (isManAtEdge(pos))
				return escapeMinute;
			for (int d = 0; d < 4; ++d) {
				int nx = x + dx[d];
				int ny = y + dy[d];
				if (ny < 0 || ny >= r || nx < 0 || nx >= c)
					continue;
				int nposState = posStates[ny][nx];
				if (pos.state == VISITED) {
					if (nposState != EMPTY)
						continue;
					posStates[ny][nx] = VISITED;
					posQ.add(new Position(nx, ny, VISITED));
				} else {
					if (nposState != EMPTY && nposState != VISITED)
						continue;
					posStates[ny][nx] = FIRE;
					posQ.add(new Position(nx, ny, FIRE));
				}
			}
		}
	} while (!posQ.isEmpty());
	return -1;
}

static boolean isManAtEdge(Position pos) {
	if (pos.state != VISITED)
		return false;
	return pos.y == 0 || pos.y == r - 1 || pos.x == 0 || pos.x == c - 1;
}

static class Position {
	int x;
	int y;
	int state;

	Position(int x, int y, int state) {
		this.x = x;
		this.y = y;
		this.state = state;
	}
}

출처:https://www.acmicpc.net/problem/4179