public int shortestDistance(int[][] grid) {
    int rows = grid.length, cols = grid[0].length; 
    int[][] canReach = new int[rows][cols];
    int[][] distance = new int[rows][cols];
    if(grid == null || grid.length == 0 || grid[0].length == 0){
        return -1;
    }
    int totalBuilding = 0; 
    for(int i = 0; i < rows; i++){
        for(int j = 0; j < cols; j++){
            if(grid[i][j] == 1){
                totalBuilding++;
                bfs(grid, i, j, canReach, distance);  
            }
        }
    }
   
    int minDist = Integer.MAX_VALUE;
    for(int i = 0; i < rows; i++){
        for(int j = 0; j < cols; j++){
            if(canReach[i][j] == totalBuilding
              &&
              distance[i][j] < minDist){
                minDist = distance[i][j];
            }
        }
    }
    return minDist == Integer.MAX_VALUE ? -1 : minDist;
    
    
}

private void bfs(int[][] grid, int row, int col, int[][] canReach, int[][] distance){
    int rows = grid.length, cols = grid[0].length;
    Queue<int[]> q = new LinkedList<>();
    boolean[][] visited = new boolean[rows][cols];
    int dist = 0; 
    q.offer(new int[] {row, col});
    visited[row][col] = true;           
    while(!q.isEmpty()){
        dist ++;
        int size = q.size();
        for(int i = 0; i < size; i++){
           int[] cur = q.poll();
            for(int d = 0; d < 4; d++){
                int rr = rowDir[d] + cur[0];
                int cc = colDir[d] + cur[1];
                
                if(!isValid(grid, rr, cc, visited)) continue;
                
                visited[rr][cc] = true;
                q.offer(new int[]{rr, cc});
                distance[rr][cc] += dist;
                canReach[rr][cc]++;
            }
        }
    }
}

private boolean isValid(int[][] grid, int rr, int cc, boolean[][] visited){
    if(rr < 0 || rr > grid.length - 1 || cc < 0 || cc > grid[0].length - 1){
        return false;
    }
     if(visited[rr][cc]) return false;
    if(grid[rr][cc] != 0){
        return false;
    }
   
    return true;
}