286 Walls and Gates

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

-1 - A wall or an obstacle.
- 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.

For example, given the 2D grid:
INF  -1  0  INF
INF INF INF  -1
INF  -1 INF  -1
 -1 INF INF
After running your function, the 2D grid should be:
 -1   0   1
  2   1  -1
 -1   2  -1
 -1   3   4

实际上就是找每个房间到最近的门的距离，我们从每个门开始，广度优先搜索并记录层数就行了。如果某个房间之前被标记过距离，那就选择这个距离和当前距离中较小的那个。这题要注意剪枝，如果下一步是门或者下一步是墙或者下一步已经访问过了，就不要加入队列中。否则会超时。