Walls and Gates

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- Tag: #bfs

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 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.

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

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

## https://code.dennyzhang.com/walls-and-gates ## Basic Ideas: BFS ## For each 0, and configure the values to the level number ## If the neighbors is 0 or value equals current level, skip them ## ## Complexity: Time O(?), Space O(n^2) class Solution: def wallsAndGates(self, rooms): """ :type rooms: List[List[int]] :rtype: void Do not return anything, modify rooms in-place instead. """ import collections row_count = len(rooms) if row_count == 0: return col_count = len(rooms[0]) for i in range(row_count): for j in range(col_count): if rooms[i][j] == 0: queue = collections.deque([(i, j)]) level = 0 while len(queue) != 0: level += 1 for k in range(len(queue)): (i1, j1) = queue.popleft() # get the neighbors for (ik, jk) in [(0, 1), (0, -1), (1, 0), (-1, 0)]: i2, j2 = i1+ik,j1+jk if i2<0 or i2>=row_count \ or j2<0 or j2>=col_count: continue if rooms[i2][j2] <= level: continue rooms[i2][j2] = level queue.append((i2, j2))