Create nearest-exit-from-entrance-in-maze.py

Author: kamyu104

Python/nearest-exit-from-entrance-in-maze.py

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+# Time:  O(m * n)
+# Space: O(m + n)
+
+# bi-bfs solution
+class Solution(object):
+    def nearestExit(self, maze, entrance):
+        """
+        :type maze: List[List[str]]
+        :type entrance: List[int]
+        :rtype: int
+        """
+        directions = [(0, 1), (1, 0), (0, -1), (-1, 0)]
+        visited = ' '
+        entrance = tuple(entrance)
+        left = set([entrance])
+        right = set([(r, 0) for r in xrange(len(maze)-1) if maze[r][0] == '.' and (r, 0) != entrance] +
+                    [(len(maze)-1, c) for c in xrange(len(maze[0])-1) if maze[len(maze)-1][c] == '.' and (len(maze)-1, c) != entrance] +
+                    [(r, len(maze[0])-1) for r in reversed(xrange(1, len(maze))) if maze[r][len(maze[0])-1] == '.' and (r, len(maze[0])-1) != entrance] +
+                    [(0, c) for c in reversed(xrange(1, len(maze[0]))) if maze[0][c] == '.' and (0, c) != entrance])
+        steps = 0
+        while left:
+            for (r, c) in left:
+                maze[r][c] = visited
+            new_left = set()
+            for (r, c) in left:
+                if (r, c) in right: 
+                    return steps
+                for dr, dc in directions:
+                    nr, nc = r+dr, c+dc
+                    if not (0 <= nr < len(maze) and
+                            0 <= nc < len(maze[0]) and
+                            maze[nr][nc] == '.'):
+                        continue
+                    new_left.add((nr, nc))
+            left = new_left
+            steps += 1
+            if len(left) > len(right): 
+                left, right = right, left
+        return -1
+
+
+# Time:  O(m * n)
+# Space: O(m + n)
+# bfs solution
+class Solution2(object):
+    def nearestExit(self, maze, entrance):
+        """
+        :type maze: List[List[str]]
+        :type entrance: List[int]
+        :rtype: int
+        """
+        directions = [(0, 1), (1, 0), (0, -1), (-1, 0)]
+        visited = ' '
+        entrance = tuple(entrance)
+        maze[entrance[0]][entrance[1]] = visited
+        q = [(entrance, 0)]
+        while q:
+            new_q = []
+            for (r, c), step in q:
+                if (r, c) != entrance and \
+                   (r in (0, len(maze)-1) or c in (0, len(maze[0])-1)):
+                    return step
+                for dr, dc in directions:
+                    nr, nc = r+dr, c+dc
+                    if not (0 <= nr < len(maze) and
+                            0 <= nc < len(maze[0]) and
+                            maze[nr][nc] == '.'):
+                        continue
+                    maze[nr][nc] = visited
+                    q.append(((nr, nc), step+1))
+            q = new_q
+        return -1