Leetcode: Walls and Gates

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.

A DFS Solution:

public class Solution {
    public void wallsAndGates(int[][] rooms) {
        if (rooms == null || rooms.length == 0) {
            return;
        }
        
        int m = rooms.length;
        int n = rooms[0].length;
        
        boolean[][] visited = new boolean[m][n];
        
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (rooms[i][j] == 0) {
                    wallsAndGatesHelper(i, j, 0, visited, rooms);
                }
            }
        }
    }
    
    private void wallsAndGatesHelper(int row, int col, int distance, boolean[][] visited, int[][] rooms) {
        int rows = rooms.length;
        int cols = rooms[0].length;
        
        if (row < 0 || row >= rows || col < 0 || col >= cols) {
            return;
        }
        
        // visited
        if (visited[row][col]) {
            return;
        }
        
        // Is wall?
        if (rooms[row][col] == -1) {
            return;
        }
        
        // Distance greater than current
        if (distance > rooms[row][col]) {
            return;
        }
        
        
        // Mark as visited
        visited[row][col] = true;
        
        if (distance < rooms[row][col]) {
            rooms[row][col] = distance;
        }
        
        // go up, down, left and right
        wallsAndGatesHelper(row - 1, col, distance + 1, visited, rooms);
        wallsAndGatesHelper(row + 1, col, distance + 1, visited, rooms);
        wallsAndGatesHelper(row, col - 1, distance + 1, visited, rooms);
        wallsAndGatesHelper(row, col + 1, distance + 1, visited, rooms);
        
        // Mark as unvisited
        visited[row][col] = false;
    }
}

A BFS Solution:

public class Solution {
    public void wallsAndGates(int[][] rooms) {
        if (rooms == null || rooms.length == 0) {
            return;
        }
        
        int m = rooms.length;
        int n = rooms[0].length;
        
        Queue<Integer> queue = new LinkedList<>();
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (rooms[i][j] == 0) {
                    wallsAndGatesHelper(i, j, 0, rooms, queue);
                }
            }
        }
    }
    
    private void wallsAndGatesHelper(int row, int col, int distance, int[][] rooms, Queue<Integer> queue) {
        fill(row, col, distance, rooms, queue);
        
        int m = rooms.length;
        int n = rooms[0].length;
        
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                int cord = queue.poll();
                int x = cord / n;
                int y = cord % n;
            
                fill(x - 1, y, distance + 1, rooms, queue);
                fill(x + 1, y, distance + 1, rooms, queue);
                fill(x, y - 1, distance + 1, rooms, queue);
                fill(x, y + 1, distance + 1, rooms, queue);
            
            }
            distance++;
        }
    }
    
    private void fill (int row, int col, int distance, int[][] rooms, Queue<Integer> queue) {
        int m = rooms.length;
        int n = rooms[0].length;
        
        if (row < 0 || row >= m || col < 0 || col >= n) {
            return;
        }
        
        if (rooms[row][col] == -1) {
            return;
        }
        
        if (distance > rooms[row][col]) {
            return;
        }
        
        if (distance < rooms[row][col]) {
            rooms[row][col] = distance;
        }
        
        int cord = row * n + col;
        queue.offer(cord);
    }
}