1 public class Solution { 2     public int shortestDistance(int[][] grid) { 3         if (grid.length == 0 || grid[0].length == 0) { 4             return -1; 5         } 6          7         int[] shift = new int[]{0, 1, 0, -1, 0}; 8         int[][] distance = new int[grid.length][grid[0].length]; 9         int[][] reach = new int[grid.length][grid[0].length];10         int numBuildings = 0;11         for (int i = 0; i < grid.length; i++) {12             for (int j = 0; j < grid[0].length; j++) {13                 if (grid[i][j] == 1) {14                     int level = 1;15                     numBuildings++;16                     boolean[][] visited = new boolean[grid.length][grid[0].length];17                     Queue
     
       queue = new LinkedList<>();18                     queue.offer(new int[]{i, j});19                     while (!queue.isEmpty()) {20                         int size = queue.size();21                         for (int k = 0; k < size; k++) {22                             int[] current = queue.poll();23                             for (int l = 0; l < 4; l++) {24                                 int x = current[0] + shift[l];25                                 int y = current[1] + shift[l + 1];26                                 if (x >= 0 && x < grid.length && y >= 0 && y < grid[0].length && !visited[x][y] && grid[x][y] == 0) {27                                     visited[x][y] = true;28                                     distance[x][y] += level;29                                     reach[x][y]++;30                                     queue.offer(new int[]{x, y});31                                 }32                             }33                         }34                         level++;35                     }36                 }37             }38         }39         40         int result = Integer.MAX_VALUE;41         for (int i = 0; i < grid.length; i++) {42             for (int j = 0; j < grid[0].length; j++) {43                 if (grid[i][j] == 0 && reach[i][j] == numBuildings) {44                     result = Math.min(result, distance[i][j]);45                 }46             }47         }48         return result == Integer.MAX_VALUE ? -1 : result;49     }50 }
     

Basic idea:

1. starting from each element expanding to all map. Find a 0, add one to reach, and level traversal to.

2. Go through each element again. Find the smallest distances.