You are given a m x n 2D grid initialized with these three possible values.
-1 - A wall or an obstacle.0 - 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 |
| 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
原文：http://www.cnblogs.com/joycelee/p/5274422.html