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

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.

For 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

 public class Solution {

     public void wallsAndGates(int[][] rooms) {

         if (rooms == null || rooms.length == ) return;

         int m = rooms.length, n = rooms[].length;

         for (int i = ; i < m; i++) {

             for (int j = ; j < n; j++) {

                 if (rooms[i][j] == ) {

                     helper(i, j, , rooms);

                 }

             }

         }

     }

     private void helper(int row, int col, int distance, int[][] rooms) {

         int rows = rooms.length, cols = rooms[].length;

         if (row <  || row >= rows || col <  || col >= cols || rooms[row][col] == -) return;

         if (distance > rooms[row][col]) return;

         if (distance < rooms[row][col]) {

             rooms[row][col] = distance;

         }

         helper(row - , col, distance + , rooms);

         helper(row + , col, distance + , rooms);

         helper(row, col - , distance + , rooms);

         helper(row, col + , distance + , rooms);

     }

 }

 public class Solution {

     public void wallsAndGates(int[][] rooms) {

         Queue<int[]> queue = new LinkedList<int[]>();

         int rows = rooms.length;

         if (rows == ) {

             return;

         }

         int cols = rooms[].length;

         // 找出所有BFS的起始点

         for (int i = ; i < rows; i++) {

             for (int j = ; j < cols; j++) {

                 if (rooms[i][j] == ) {

                     queue.offer(new int[]{i, j});

                 }

             }

         }

         // 定义下一步的位置

         int[][] dirs = {{-, }, {, }, {, -}, {, }};

         // 开始BFS

         while (!queue.isEmpty()) {

             int[] top = queue.poll();

             for (int k = ; k < dirs.length; k++) {

                 int x = top[] + dirs[k][];

                 int y = top[] + dirs[k][];

                 if (x >=  && x < rows && y >=  && y < cols && rooms[x][y] == Integer.MAX_VALUE) {

                     rooms[x][y] = rooms[top[]][top[]] + ;

                     queue.add(new int[]{x, y});

                 }

             }

         }

     }

 }