You are given a m x n 2D grid initialized with these three possible values.</p> -1 - A wall or an obstacle.</p> 0 - A gate.</p> 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.</p> 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.

public 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) {
                    helper(rooms, i, j);
                }
            }
        }
    }
    
    public void helper(int[][] rooms, int i, int j) {
        int[] dx = {-1, 0, 0, 1};
        int[] dy = {0, -1, 1, 0};
        for (int k = 0; k < 4; k++) {
            int x = i + dx[k], y = j + dy[k];
            if (x >= 0 && x < rooms.length && y >= 0 && y < rooms[0].length && rooms[x][y] > rooms[i][j] + 1) {
                //rooms[x][y] > rooms[i][j] + 1 确保只有INF才会被修改, 0 和 -1均不变
                rooms[x][y] = rooms[i][j] + 1;
                helper(rooms, x, y);
            }
        }
    }
}