private void exploreNeighbours(int r, int c) {
        for(int i = 0; i < 4; i++){
            int rr = r + dr[i];
            int cc = c + dc[i];
            //Skip out of bounds locations
            if(rr < 0 || cc < 0)continue;
            if(rr >= R || cc >= C)continue;
            //Skip visited cells
            if(visited[rr][cc])continue;
            //Skip invalid cells
            if(m[rr][cc] == m[r][c])continue;
            //Add neighbour to queue
            rq.add(rr);
            cq.add(cc);
            //Set neighbour to visited = true and add to next layer
            visited[rr][cc] = true;
            nodes_in_next_layer++;
        }
    }