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[LeetCode] 286. Walls and Gates

You are given an m x n grid rooms 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.

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

墙与门。

题意是给一个二维矩阵，里面的 -1 代表墙，0 代表门，INF 代表一个空的房间。请改写所有的 INF，表明每个 INF 到最近的门的距离。

这一题是带障碍物的 flood fill类的题目，既然是问最短距离，所以这个题目应该还是偏 BFS 做。还有一题思路比较接近的是542题，也是在矩阵内通过已知的一些距离去累加起来找未知坐标的距离。还有一道题也比较类似，也是带障碍物的 flood fill 类型的题目，1730题。

具体思路是先找到矩阵中的 0，把这些 0 的坐标放入 queue。从 queue 中弹出这些0的时候，往四个方向扫描，看看这些 0 的周围是否有 INF。如果有，则这些 INF 就有一个具体的距离了，把这些有了距离的 INF 再放入 queue。这些 INF 有了具体的距离之后，别的 INF 也就有机会被计算出具体的距离了。

时间O(mn)

空间O(mn)

Java实现

 1 class Solution {
 2     public void wallsAndGates(int[][] rooms) {
 3         // corner case
 4         if (rooms == null || rooms.length == 0) {
 5             return;
 6         }
 7 
 8         // normal case
 9         Queue<int[]> queue = new LinkedList<>();
10         for (int i = 0; i < rooms.length; i++) {
11             for (int j = 0; j < rooms[0].length; j++) {
12                 if (rooms[i][j] == 0) {
13                     queue.offer(new int[] { i, j });
14                 }
15             }
16         }
17         while (!queue.isEmpty()) {
18             int[] cur = queue.poll();
19             int r = cur[0];
20             int c = cur[1];
21             if (r > 0 && rooms[r - 1][c] == Integer.MAX_VALUE) {
22                 rooms[r - 1][c] = rooms[r][c] + 1;
23                 queue.offer(new int[] { r - 1, c });
24             }
25             if (c > 0 && rooms[r][c - 1] == Integer.MAX_VALUE) {
26                 rooms[r][c - 1] = rooms[r][c] + 1;
27                 queue.offer(new int[] { r, c - 1 });
28             }
29             if (r < rooms.length - 1 && rooms[r + 1][c] == Integer.MAX_VALUE) {
30                 rooms[r + 1][c] = rooms[r][c] + 1;
31                 queue.offer(new int[] { r + 1, c });
32             }
33             if (c < rooms[0].length - 1 && rooms[r][c + 1] == Integer.MAX_VALUE) {
34                 rooms[r][c + 1] = rooms[r][c] + 1;
35                 queue.offer(new int[] { r, c + 1 });
36             }
37         }
38     }
39 }