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

1. -1 - A wall or an obstacle.
2. 0 - A gate.
3. 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.
   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

一刷
题解：以所有的gates为起始点做BFS

public class Solution {
    public void wallsAndGates(int[][] rooms) {
        if(rooms.length == 0 || rooms[0].length == 0) return;
        Queue<int[]> 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.add(new int[]{i, j});
            }
        }
        
        while(!queue.isEmpty()){
            int[] top = queue.remove();
            int row = top[0], col = top[1];
            if(row>0 && rooms[row-1][col] == Integer.MAX_VALUE){
                rooms[row-1][col] = rooms[row][col] + 1;
                queue.add(new int[]{row-1, col});
            }
            if(row<rooms.length-1 && rooms[row+1][col] == Integer.MAX_VALUE){
                rooms[row+1][col] = rooms[row][col] + 1;
                queue.add(new int[]{row+1, col});
            }
            if(col>0 && rooms[row][col-1] == Integer.MAX_VALUE){
                rooms[row][col-1] = rooms[row][col] + 1;
                queue.add(new int[]{row, col-1});
            }
            if(col<rooms[0].length-1 && rooms[row][col+1] == Integer.MAX_VALUE){
                rooms[row][col+1] = rooms[row][col] + 1;
                queue.add(new int[]{row, col+1});
            }
        }
    }
}

二刷
从gates出发，BFS

class Solution {
    public void wallsAndGates(int[][] rooms) {
        int m = rooms.length;
        if(m == 0) return;
        int n = rooms[0].length;
        Queue<int[]> queue = new LinkedList<>();
        for(int i=0; i<m; i++){
            for(int j=0; j<n; j++){
                if(rooms[i][j] == 0) queue.add(new int[]{i, j});
            }
        }
        
        while(!queue.isEmpty()){
            int[] cur = queue.poll();
            int row = cur[0];
            int col = cur[1];
            if(row-1>=0 && rooms[row-1][col] == Integer.MAX_VALUE){
                rooms[row-1][col] = rooms[row][col] + 1;
                queue.add(new int[]{row-1, col});
            }
            if(row+1<m && rooms[row+1][col] == Integer.MAX_VALUE){
                rooms[row+1][col] = rooms[row][col] + 1;
                queue.add(new int[]{row+1, col});
            }
            if(col-1>=0 && rooms[row][col-1] == Integer.MAX_VALUE){
                rooms[row][col-1] = rooms[row][col] + 1;
                queue.add(new int[]{row, col-1});
            }
            if(col+1<n && rooms[row][col+1] == Integer.MAX_VALUE){
                rooms[row][col+1] = rooms[row][col] + 1;
                queue.add(new int[]{row, col+1});
            }
        }
    }
}

三刷

class Solution {
    class Cell{
        int x;
        int y;
        Cell(int x, int y){
            this.x = x;
            this.y = y;
        }
    }
    
    public void wallsAndGates(int[][] rooms) {
        if(rooms == null || rooms.length == 0 || rooms[0].length == 0) return;
        int[][] dirs = {{1,0},{-1,0},{0,1},{0,-1}};
        Deque<Cell> queue = new LinkedList<>();
        int m = rooms.length, n = rooms[0].length;
        for(int i=0; i<m; i++){
            for(int j=0; j<n; j++){
                if(rooms[i][j] == 0){
                    Cell cur = new Cell(i, j);
                    queue.add(cur);
                    //System.out.println(cur.x+"/"+cur.y+"/"+rooms[cur.x][cur.y]);
                }
            }
        }
        while(!queue.isEmpty()){
            Cell cur = queue.poll();
            //System.out.println(cur.x+"/"+cur.y);
            for(int[] dir : dirs){
                int x = cur.x+dir[0];
                int y = cur.y+dir[1];
                if(x>=0 && x<m && y>=0 && y<n && rooms[x][y] == Integer.MAX_VALUE) {
                    rooms[x][y] = rooms[cur.x][cur.y]+1;
                    queue.add(new Cell(x,y));
                    //System.out.println(x+"/"+y+"/"+rooms[x][y]);
                }
            }
        }
    }
}