286. Walls and Gates

You are given am x n2D 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 value231- 1 = 2147483647to representINFas you may assume that the distance to a gate is less than2147483647.
Fill each empty room with the distance to itsnearestgate. If it is impossible to reach a gate, it should be filled withINF.
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

BFS， 把0 一起都放到队列里， 然后开始循化队列，如果 队列里面的数字旁边遇到INF， 则 把INF的值更改为队列值+1， 再把这个被更新的INF放到队列里面。