286 Walls and Gates *** 注意感受DFS

You are given a m x n 2D grid initialized with these three possible values.

-1 - A wall or an obstacle.
0 - A gate.
INF - Infinity means an empty room. We use the value 231 - 1 = 2147483647 to represent INF as you may assume that the distance to a gate is less than 2147483647.
Fill each empty room with the distance to its nearest gate. If it is impossible to reach a gate, it should be filled with INF.

Example: 

Given the 2D grid:

INF  -1  0  INF
INF INF INF  -1
INF  -1 INF  -1
  0  -1 INF INF
After running your function, the 2D grid should be:

  3  -1   0   1
  2   2   1  -1
  1  -1   2  -1
  0  -1   3   4

BFS

class Solution {
    public void wallsAndGates(int[][] rooms) {
        int INF = 2147483647;
        if (rooms == null || rooms.length == 0 || rooms[0].length == 0) {
            return;
        }
        int rows = rooms.length;
        int cols = rooms[0].length;
        Queue<Cell> queue = new LinkedList<>();
        for (int i = 0; i < rooms.length; i++) {
            for(int j = 0; j < rooms[0].length; j++) {
                if(rooms[i][j] == 0) {
                    queue.offer(new Cell(i, j , 0));
                }
            }
        }
        int[] dx = {0, 0, 1, -1};
        int[] dy = {-1, 1, 0, 0};
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int j = 0; j < size; j++) {
                Cell head = queue.poll();
                for (int i = 0; i < 4; i++) {
                    int newX = head.x + dx[i];
                    int newY = head.y + dy[i];
                 if (newX < 0 || newX >= rows || newY < 0 || newY >= cols|| rooms[newX][newY] == -1 ||rooms[newX][newY] == 0) {
                        continue;
                    }
                    if (rooms[newX][newY] == INF) {
                        Cell newCell = new Cell(newX, newY, head.level + 1);
                        rooms[newX][newY] = head.level + 1;
                        queue.offer(newCell);
                    } 
                }
            }
        }
    }
    class Cell {
        int x;
        int y;
        int level;
        public Cell (int xx, int yy, int level) {
            this.x = xx;
            this.y = yy;
            this.level = level;
        }
    }
}

DFS

class Solution {
    public void wallsAndGates(int[][] rooms) {
        if (rooms == null || rooms.length == 0) {
            return;
        }
        for (int i = 0; i < rooms.length; i++) {
            for (int j = 0; j < rooms[0].length; j++) {
                if (rooms[i][j] == 0) {
                    dfs(rooms, i, j, 0);
                }
            }
        }
    }
    public void dfs(int[][] rooms, int x, int y, int level) {
        if (x < 0 || x >= rooms.length || y < 0 || y >= rooms[0].length || rooms[x][y] < level) {
            return;
        }
        rooms[x][y] = level;
        dfs(rooms, x + 1, y, level + 1);
        dfs(rooms, x - 1, y, level + 1);
        dfs(rooms, x, y + 1, level + 1);
        dfs(rooms, x, y - 1, level + 1);
    } 
}