WallsAndGates.java

package medium;

import java.util.*;

/**
 * Created by fishercoder1534 on 9/30/16.
 */
public class WallsAndGates {
    class BFS_solution_without_queue{

        int[] dirs = new int[]{0,1,0,-1,0};
        public void wallsAndGates(int[][] rooms) {
            if(rooms == null || rooms.length == 0 || rooms[0].length == 0) return;
            int m = rooms.length, n = rooms[0].length;
            for(int i = 0; i < m; i++){
                for(int j = 0; j < n; j++){
                    if(rooms[i][j] == 0) bfs(rooms, i, j, m, n);
                }
            }
        }

        void bfs(int[][] rooms, int i, int j, int m, int n){
            for(int k = 0; k < 4; k++){
                int x = dirs[k]+i;
                int y = dirs[k+1]+j;
                if(x >= 0 && y >= 0 && x < m && y < n && rooms[x][y] > rooms[i][j]+1) {
                    rooms[x][y] = rooms[i][j]+1;
                    bfs(rooms, x, y, m, n);
                }
            }
        }

    }

    class BFS_solution_with_a_queue{

        //push all gates into the queue first, and then put all its neighbours into the queue with one distance to the gate, then continue to push the rest of the nodes into the queue, and put all their neighbours into the queue with the nodes' value plus one until the queue is empty
        int[] dirs = new int[]{0,1,0,-1,0};
        public void wallsAndGates(int[][] rooms) {
            if(rooms == null || rooms.length == 0 || rooms[0].length == 0) return;
            int m = rooms.length, n = rooms[0].length;
            Queue<int[]> queue = new LinkedList();
            for(int i = 0; i < m; i++){
                for(int j = 0; j < n; j++){
                    if(rooms[i][j] == 0) queue.offer(new int[]{i,j});
                }
            }

            while(!queue.isEmpty()){
                int[] curr = queue.poll();
                for(int k = 0; k < 4; k++){
                    int x = curr[0]+dirs[k];
                    int y = curr[1]+dirs[k+1];
                    if(x >= 0 && x < m && y >= 0 && y < n && rooms[x][y] == Integer.MAX_VALUE) {
                        rooms[x][y] = rooms[curr[0]][curr[1]]+1;
                        queue.offer(new int[]{x,y});
                    }
                }
            }
        }

    }
}