text: ##-------------------------------------------------------------------
  """
  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 2^31 - 1 = 2147483647 to represent INF 
           as you may assume that the distance to a gate is less than 2147483647.
  Fill the empty room with 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
  ##-------------------------------------------------------------------
  """
  
  
  def walls_and_gates(rooms):
      for i in range(len(rooms)):
          for j in range(len(rooms[0])):
              if rooms[i][j] == 0:
                  dfs(rooms, i, j, 0)
  
  
  def dfs(rooms, i, j, depth):
      if (i < 0 or i >= len(rooms)) or (j < 0 or j >= len(rooms[0])):
          return  # out of bounds
      if rooms[i][j] < depth:
          return  # crossed
      rooms[i][j] = depth
      dfs(rooms, i + 1, j, depth + 1)
      dfs(rooms, i - 1, j, depth + 1)
      dfs(rooms, i, j + 1, depth + 1)
      dfs(rooms, i, j - 1, depth + 1)
  