WallsAndGates.java

package breadth_first_search;

import java.util.ArrayDeque;
import java.util.Queue;

/**
 * Created by gouthamvidyapradhan on 26/12/2017.
 * You are given a m x n 2D grid 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.

 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

 Solution: O(n x m): Treat each coordinate of grid with 0 as a source and destination as the coordinate of 2147483647
 and perform a multi-sources BFS from each source.
 */
public class WallsAndGates {

    private static final int[] R = {0, 0, 1, -1};
    private static final int[] C = {1, -1, 0, 0};

    private class Cell{
        int r, c;
        Cell(int r, int c){
            this.r = r;
            this.c = c;
        }
    }

    /**
     * Main method
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception{
        int[][] A = {{Integer.MAX_VALUE, -1, 0, Integer.MAX_VALUE},
                {Integer.MAX_VALUE, Integer.MAX_VALUE, Integer.MAX_VALUE, -1},
                {Integer.MAX_VALUE, -1, Integer.MAX_VALUE, -1},
                {0, -1, Integer.MAX_VALUE, Integer.MAX_VALUE}};
        new WallsAndGates().wallsAndGates(A);
    }

    public void wallsAndGates(int[][] rooms) {
        Queue<Cell> queue = new ArrayDeque<>();
        for(int i = 0; i < rooms.length; i ++){
            for(int j = 0; j < rooms[0].length; j ++){
                if(rooms[i][j] == 0){ //treat each co-ordinates of gate as a source
                    Cell cell = new Cell(i, j);
                    queue.offer(cell);
                }
            }
        }
        while(!queue.isEmpty()){
            Cell top = queue.poll();
            for(int i = 0; i < 4; i++){
                int newR = top.r + R[i];
                int newC = top.c + C[i];
                if(newR >= 0 && newC >= 0 && newR < rooms.length && newC < rooms[0].length){
                    if(rooms[newR][newC] == Integer.MAX_VALUE){
                        rooms[newR][newC] = rooms[top.r][top.c] + 1;
                        queue.offer(new Cell(newR, newC));
                    }
                }
            }
        }
    }
}