LeetCode – Shortest Distance from All Buildings (Java)

public int shortestDistance(int[][] grid) {
    int[][] distance = new int[grid.length][grid[0].length];
    int[][] reach = new int[grid.length][grid[0].length];
 
 
    int numBuilding = 0;
    for (int i = 0; i < grid.length; i++) {
        for (int j = 0; j < grid[0].length; j++) {
            if (grid[i][j] == 1) {
                helper(grid, distance, reach, i, j);
                numBuilding++;
            }
        }
    }
 
    int result = Integer.MAX_VALUE;
    for (int i = 0; i < grid.length; i++) {
        for (int j = 0; j < grid[0].length; j++) {
            if (grid[i][j] == 0 && reach[i][j] == numBuilding) {
                result = Math.min(result, distance[i][j]);
            }
        }
    }
 
    return result == Integer.MAX_VALUE ? -1 : result;
}
 
private void helper(int[][] grid, int[][] distance, int[][] reach, int i, int j) {
 
    int[] dx = {-1, 0, 1, 0};
    int[] dy = {0, 1, 0, -1};
 
    //two queue, one for direction, one for distance tracking
    LinkedList<int[]> q = new LinkedList<>();
    LinkedList<Integer> qDist = new LinkedList<>();
 
    q.offer(new int[]{i, j});
    qDist.offer(1);
 
    while (!q.isEmpty()) {
        int[] head = q.poll();
        int dis = qDist.poll();
 
        for (int k = 0; k < 4; k++) {
            int x = head[0] + dx[k];
            int y = head[1] + dy[k];
 
            if (x >= 0 && y >= 0 && x < grid.length && y < grid[0].length && grid[x][y] == 0) {
                grid[x][y] = -1;
 
                q.offer(new int[]{x, y});
                qDist.offer(dis + 1);
 
                distance[x][y] += dis;
                reach[x][y]++;
            }
        }
    }
 
    for (int m = 0; m < grid.length; m++) {
        for (int n = 0; n < grid[0].length; n++) {
            if (grid[m][n] == -1) {
                grid[m][n] = 0;
            }
        }
    }
}