# 0286. Walls and Gates

<https://leetcode.com/problems/walls-and-gates>

## Description

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 1:**

![](https://assets.leetcode.com/uploads/2021/01/03/grid.jpg)

```
**Input:** rooms = [[2147483647,-1,0,2147483647],[2147483647,2147483647,2147483647,-1],[2147483647,-1,2147483647,-1],[0,-1,2147483647,2147483647]]
**Output:** [[3,-1,0,1],[2,2,1,-1],[1,-1,2,-1],[0,-1,3,4]]
```

**Example 2:**

```
**Input:** rooms = [[-1]]
**Output:** [[-1]]
```

**Example 3:**

```
**Input:** rooms = [[2147483647]]
**Output:** [[2147483647]]
```

**Example 4:**

```
**Input:** rooms = [[0]]
**Output:** [[0]]
```

**Constraints:**

* `m == rooms.length`
* `n == rooms[i].length`
* `1 <= m, n <= 250`
* `rooms[i][j]` is `-1`, `0`, or `231 - 1`.

## ac1: BFS

```java
class Solution {
    public void wallsAndGates(int[][] rooms) {
        // edge case
        if (rooms.length == 0 || rooms[0].length == 0) return;

        Queue<int[]> q = new LinkedList<int[]>();
        int row = rooms.length;
        int col = rooms[0].length;

        // walk, get gates
        for (int r = 0; r < row; r++) {
            for (int c = 0; c < col; c++) {
                if (rooms[r][c] == 0) q.offer(new int[] {r, c});
            }
        }

        // BFS
        int layer = 0;
        while (!q.isEmpty()) {
            layer++;
            int len = q.size();
            for (int i = 0; i < len; i++) {
                int[] head = q.poll();
                int r = head[0]; int c = head[1];

                if (r < row-1 && rooms[r+1][c] > layer) {
                    rooms[r+1][c] = layer;
                    q.offer(new int[] {r+1, c});
                }
                if (r > 0 && rooms[r-1][c] > layer) {
                    rooms[r-1][c] = layer;
                    q.offer(new int[] {r-1, c});
                }    
                if (c < col-1 && rooms[r][c+1] > layer){
                    rooms[r][c+1] = layer;
                    q.offer(new int[] {r, c+1});
                } 
                if (c > 0 && rooms[r][c-1] > layer) {
                    rooms[r][c-1] = layer;
                    q.offer(new int[] {r, c-1});
                }
            }
        }
    }
}

/**
walk, get 0s
BFS, if val > layer, set val = layer, offer to queue
**/
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://jaywin.gitbook.io/leetcode/solutions/0286-walls-and-gates.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.