317 Shortest Distance from All Buildings

Author: idf

317 Shortest Distance from All Buildings.py

@@ -12,6 +12,17 @@ def __init__(self):
 
     def shortestDistance(self, grid):
         """
+        BFS & collect all distance
+
+        ideas:
+        Pruning: don't use a fresh "visited" for each BFS. Instead, I walk only
+        onto the cells that were reachable from all previous buildings. From the
+        first building I only walk onto cells where grid is 0, and make them -1.
+        From the second building I only walk onto cells where grid is -1, and I
+        make them -2.
+        -1
+        -2
+        -3
         :type grid: List[List[int]]
         :rtype: int
         """
@@ -72,4 +83,4 @@ def bfs(self, grid, acc, reachable, x, y):
         [[1, 1, 1, 1, 1, 0], [0, 0, 0, 0, 0, 1], [0, 1, 1, 0, 0, 1], [1, 0, 0, 1, 0, 1], [1, 0, 1, 0, 0, 1],
          [1, 0, 0, 0, 0, 1], [0, 1, 1, 1, 1, 0]]) == 88
     assert Solution().shortestDistance([[1, 2, 0]]) == -1
-    assert Solution().shortestDistance([[1, 0, 2, 0, 1], [0, 0, 0, 0, 0], [0, 0, 1, 0, 0]]) == 7
+    assert Solution().shortestDistance([[1, 0, 2, 0, 1], [0, 0, 0, 0, 0], [0, 0, 1, 0, 0]]) == 7