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Walls and gates- Find shortest distances between rooms and gates

Leetcode: Walls and Gates

You are given a m x n 2D grid initialized with these three possible values.
  1. -1 - A wall or an obstacle.
  2. 0 - A gate.
  3. 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 than2147483647.
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
Understand the problem:

It is very classic backtracking problem. We can start from each gate (0 point), and searching for its neighbors. We can either use DFS or BFS solution.

 Below is a DFS solution-

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