Fill each empty room in a grid with the distance to its nearest gate. INF=empty room, -1=wall, 0=gate.
Input: [[INF,-1,0,INF],[INF,INF,INF,-1],[INF,-1,INF,-1],[0,-1,INF,INF]] Output: [[3,-1,0,1],[2,2,1,-1],[1,-1,2,-1],[0,-1,3,4]]
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