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