Leetcode: Walls and Gates

Walls and Gates



Similar Problems:


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.

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.


## Blog link: 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*n)
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))
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