286. Walls and Gates

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https://leetcode.com/problems/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 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.
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
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Time - O( m * n )
Space - O( m * n ) - Queue size - worst case - all gates
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Related problems
shortest-distance-from-all-buildings
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