Leetcode: Walls and Gates

Question

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
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My first try

if there are k zero elements,
size(rooms) = (m,n)
Time complexity: O( kmn )

class Solution(object):    def wallsAndGates(self, rooms):        """        :type rooms: List[List[int]]        :rtype: void Do not return anything, modify rooms in-place instead.        """        if len(rooms)==0 or len(rooms[0])==0:            return         zero_list = []        for i in range(len(rooms)):            for j in range(len(rooms[0])):                if rooms[i][j]==0:                    zero_list.append( (i,j) )        for zero_item in zero_list:            self.traverse4zero( zero_item, rooms )    def traverse4zero(self, zero_item, rooms):        #print "abc"        bfs_list = [zero_item]        dict_direct = [(-1,0), (1,0), (0,1), (0,-1)]        visited = [ [0]*len(rooms[0]) for dummy in range(len(rooms)) ]        visited[zero_item[0]][zero_item[1]] = 1        while bfs_list!=[]:            #print bfs_list            cur_item = bfs_list[0]            bfs_list = bfs_list[1:]            self.eachdirection( rooms, cur_item, visited, bfs_list, dict_direct )        return     def eachdirection(self, rooms, cur_item, visited, bfs_list, dict_direct):        for i in range(4):            next_i = cur_item[0] + dict_direct[i][0]            next_j = cur_item[1] + dict_direct[i][1]            if next_i<0 or next_i>=len(rooms) \                or next_j<0 or next_j>=len(rooms[0]) \                or rooms[next_i][next_j]==0 \                or rooms[next_i][next_j]==-1 \                or visited[next_i][next_j]==1 \                or rooms[cur_item[0]][cur_item[1]]+1 >= rooms[next_i][next_j]:                    continue            rooms[next_i][next_j] = min( rooms[next_i][next_j], \                        rooms[cur_item[0]][cur_item[1]]+1 )            bfs_list.append( (next_i, next_j) )            visited[next_i][next_j] = 1        return       

Solution

Accept
Time: 540ms

class Solution(object):    def wallsAndGates(self, rooms):        """        :type rooms: List[List[int]]        :rtype: void Do not return anything, modify rooms in-place instead.        """        if len(rooms)==0 or len(rooms[0])==0:            return         zero_list = []        for i in range(len(rooms)):            for j in range(len(rooms[0])):                if rooms[i][j]==0:                    self.traverse4zero( (i,j), rooms )    def traverse4zero(self, zero_item, rooms):        bfs_list = [zero_item]        dict_direct = [(-1,0), (1,0), (0,1), (0,-1)]        while bfs_list!=[]:            cur_item = bfs_list[0]            bfs_list = bfs_list[1:]            for i in range(4):                next_i = cur_item[0] + dict_direct[i][0]                next_j = cur_item[1] + dict_direct[i][1]                if next_i<0 or next_i>=len(rooms) \                    or next_j<0 or next_j>=len(rooms[0]) \                    or rooms[next_i][next_j]<=0 \                    or rooms[cur_item[0]][cur_item[1]]+1 >= rooms[next_i][next_j]:                        continue                rooms[next_i][next_j] = min( rooms[next_i][next_j], \                            rooms[cur_item[0]][cur_item[1]]+1 )                bfs_list.append( (next_i, next_j) )        return

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Solution2

Accept
Time: 588ms

This solution is different from solution1. BFS is done for each zero element at the same time.

class Solution(object):    def wallsAndGates(self, rooms):        """        :type rooms: List[List[int]]        :rtype: void Do not return anything, modify rooms in-place instead.        """        if len(rooms)==0 or len(rooms[0])==0:            return         bfs_list = []        for i in range(len(rooms)):            for j in range(len(rooms[0])):                if rooms[i][j]==0:                    bfs_list.append( (i,j) )        dict_direct = [(-1,0), (1,0), (0,1), (0,-1)]        while bfs_list!=[]:            cur_item = bfs_list[0]            del bfs_list[0]             for i in range(4):                next_i = cur_item[0] + dict_direct[i][0]                next_j = cur_item[1] + dict_direct[i][1]                if next_i<0 or next_i>=len(rooms) \                    or next_j<0 or next_j>=len(rooms[0]) \                    or rooms[cur_item[0]][cur_item[1]]+1 >= rooms[next_i][next_j]:                        continue                rooms[next_i][next_j] = rooms[cur_item[0]][cur_item[1]]+1                bfs_list.append( (next_i, next_j) )        return       

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Take home message

For popping the first element of list, use

del bfs_list[0]

instead of

bfs_list = bfs_list[1:]