Walls and Gates

Posted 2020-08-14 北叶青藤

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 than2147483647.

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

Understand the problem:
It is very classic backtracking problem. We can start from each gate (0 point), and searching for its neighbors. We can either use DFS or BFS solution.

 1 public class Solution {
 2     public void wallsAndGates(int[][] rooms) {
 3         if (rooms == null || rooms.length == 0) {
 4             return;
 5         }
 6          
 7         int m = rooms.length;
 8         int n = rooms[0].length;
 9          
10         boolean[][] visited = new boolean[m][n];
11          
12         for (int i = 0; i < m; i++) {
13             for (int j = 0; j < n; j++) {
14                 if (rooms[i][j] == 0) {
15                     wallsAndGatesHelper(i, j, 0, visited, rooms);
16                 }
17             }
18         }
19     }
20      
21     private void wallsAndGatesHelper(int row, int col, int distance, boolean[][] visited, int[][] rooms) {
22         int rows = rooms.length;
23         int cols = rooms[0].length;
24          
25         if (row < 0 || row >= rows || col < 0 || col >= cols) {
26             return;
27         }
28          
29         // visited
30         if (visited[row][col]) {
31             return;
32         }
33          
34         // Is wall?
35         if (rooms[row][col] == -1) {
36             return;
37         }
38          
39         // Distance greater than current
40         if (distance > rooms[row][col]) {
41             return;
42         }
43          
44          
45         // Mark as visited
46         visited[row][col] = true;
47          
48         if (distance < rooms[row][col]) {
49             rooms[row][col] = distance;
50         }
51          
52         // go up, down, left and right
53         wallsAndGatesHelper(row - 1, col, distance + 1, visited, rooms);
54         wallsAndGatesHelper(row + 1, col, distance + 1, visited, rooms);
55         wallsAndGatesHelper(row, col - 1, distance + 1, visited, rooms);
56         wallsAndGatesHelper(row, col + 1, distance + 1, visited, rooms);
57          
58         // Mark as unvisited
59         visited[row][col] = false;
60     }
61 }

From:

http://buttercola.blogspot.com/2015/09/leetcode-walls-and-gates.html