LeetCode 286. Walls and Gates

原题链接在这里：https://leetcode.com/problems/walls-and-gates/

题目：

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

题解：

BFS, 先把所有gate加到que中。对于每一个从que中poll出来的gate，看四个方向是否有room, 若有，把room的值改正gate + 1, 在放回到que中.

Time Complexity: O(m*n). m = rooms.length, n = rooms[0].length. 每个点没有扫描超过两遍. Space: O(m*n). 

AC Java:

class Solution {
    public void wallsAndGates(int[][] rooms) {
        if(rooms == null || rooms.length == 0 || rooms[0].length == 0){
            return;
        }
        
        int m = rooms.length;
        int n = rooms[0].length;
        LinkedList<int []> que = new LinkedList<>();
        int level = 1;
        for(int i = 0; i < m; i++){
            for(int j = 0; j < n; j++){
                if(rooms[i][j] == 0){
                    que.add(new int[]{i, j});
                }
            }
        }
        
        int [][] dirs = new int[][]{{1, 0}, {0, 1}, {-1, 0}, {0, -1}};
        
        while(!que.isEmpty()){
            int size = que.size();
            while(size-- > 0){
                int [] cur = que.poll();
                for(int [] dir : dirs){
                    int x = cur[0] + dir[0];
                    int y = cur[1] + dir[1];
                    if(x < 0 || x >= m || y < 0 || y >= n || rooms[x][y] != Integer.MAX_VALUE){
                        continue;
                    }
                    
                    que.add(new int[]{x, y});
                    rooms[x][y] = level;
                }        
            }
            
            level++;
        }
    }
}

跟上Robot Room Cleaner, Rotting Oranges, Shortest Distance from All Buildings.