Problem:
Go around the Labyrinth

Explorer Taro got a floor plan of a labyrinth.
The floor of this labyrinth is in the form of a two-dimensional grid.
Each of the cells on the floor plan corresponds to a room and is indicated whether it can be entered or not.
The labyrinth has only one entrance located at the northwest corner, which is upper left on the floor plan.
There is a treasure chest in each of the rooms at other three corners, southwest, southeast, and northeast.
To get a treasure chest, Taro must move onto the room where the treasure chest is placed.

Taro starts from the entrance room and repeats moving to one of the enterable adjacent rooms in the four directions, north, south, east, or west.
He wants to collect all the three treasure chests and come back to the entrance room.
A bad news for Taro is that, the labyrinth is quite dilapidated and even for its enterable rooms except for the entrance room, floors are so fragile that, once passed over, it will collapse and the room becomes not enterable.
Determine whether it is possible to collect all the treasure chests and return to the entrance.

Input

The input consists of at most 100 datasets, each in the following format.

N M
c1,1...c1,M
...
cN,1...cN,M

The first line contains N, which is the number of rooms in the north-south direction, and M, which is the number of rooms in the east-west direction.
N and M are integers satisfying 2 ≤ N ≤ 50 and 2 ≤ M ≤ 50.
Each of the following N lines contains a string of length M.
The j-th character ci,j of the i-th string represents the state of the room (i,j), i-th from the northernmost and j-th from the westernmost;
the character is the period ('.') if the room is enterable and the number sign ('#') if the room is not enterable.
The entrance is located at (1,1), and the treasure chests are placed at (N,1), (N,M) and (1,M).
All of these four rooms are enterable.
Taro cannot go outside the given N × M rooms.

The end of the input is indicated by a line containing two zeros.

Output

For each dataset, output YES if it is possible to collect all the treasure chests and return to the entrance room, and otherwise, output NO in a single line.

Sample Input

3 3
...
.#.
...
5 5
..#..
.....
#....
.....
.....
3 8
..#.....
........
.....#..
3 5
..#..
.....
..#..
4 4
....
....
..##
..#.
0 0

Output for the Sample Input

YES
NO
YES
NO
NO

Solution in C (judge verdict: Accepted)

// AOJ 1622 Go around the Labyrinth
// 2018.2.13 bal4u

#include <stdio.h>
#include <string.h>

typedef struct { char r, c; } Q;
Q q[2000]; int top;
int h, w;
char map[53][53];
int mv4[4][2] = {{-1,0},{0,1},{1,0},{0,-1}};
int mv9[9][2] = {{-1,-1},{-1,0},{-1,1},{0,-1},{0,0},{0,1},{-1,1},{-1,0},{1,1}};
char mk[53][53];

int check()
{
	int i, r, c, nr, nc, f;

	memset(mk, 0, sizeof(mk));
	q[0].r = 1, q[0].c = 1, top = 1;
	mk[1][1] = 1;
	f = 0;
	while (top) {
		r = q[--top].r, c = q[top].c;
		if      (r == 1) { if (c == w) f++; }
		else if (r == h) {
			if      (c == 1) f++;
		    else if (c == w) f++;
		}
		if (f == 3) return 0;
		for (i = 0; i < 4; i++) {
			nr = r + mv4[i][0], nc = c + mv4[i][1];
			if (!map[nr][nc] || mk[nr][nc]) continue;
			mk[nr][nc] = 1;
			q[top].r = nr, q[top++].c = nc;
		}
	}
	return 1;
}

int main()
{
	int r, c, i, f, nr, nc;
	char buf[10], *p;

	while (fgets(buf, 10, stdin) && *buf != '0') {
		sscanf(buf, "%d%d", &h, &w);
		memset(map, 0, sizeof(map));
		for (r = 1; r <= h; r++) {
			fgets(p = map[r]+1, 52, stdin);
			c = w; while (c--) *p = (*p == '.'), p++; *p = 0;
		}

		if (check()) goto no;
		for (r = 1; r <= h; r++) for (c = 1; c <= w; c++) {
			if (r == 1 && c == 1 || r == 1 && c == w ||
				r == h && c == 1 || r == h && c == w) continue;
			if (!map[r][c]) continue;
			f = 0;
			for (i = 0; i < 9; i++) {
				nr = r + mv9[i][0], nc = c + mv9[i][1];
				if (!map[nr][nc]) { f = 1; break; }
			}
			if (f) {
				map[r][c] = 0;
				if (check()) goto no;
				map[r][c] = 1;
			}
		}
		puts("YES"); continue;
no:		puts("NO");
	}
	return 0;
}