Note:

1. There is a trick here: As it only gets the starting point, we can check each bound of the building. Also we have to have a "sign" to know when the bound is start when the bound is end.

2. REMEMBER to add 0 to the queue to avoid NPE. Also 0 represents there is no building there.

class Solution {    public List
     
       getSkyline(int[][] buildings) {        if (buildings.length == 0) {            return new ArrayList<>();        }                List
      
        result = new ArrayList<>();        List
       
         height = new ArrayList<>();        for (int[] building : buildings) {            height.add(new int[]{building[0], -building[2]});            height.add(new int[]{building[1], building[2]});        }                Collections.sort(height, (b1, b2) -> {
   return b1[0] == b2[0] ? b1[1] - b2[1] : b1[0] - b2[0];});        PriorityQueue
        
          queue = new PriorityQueue<>((i1, i2) -> (i2 - i1));        queue.offer(0);        int prev = 0;        for (int[] vertical : height) {            if (vertical[1] < 0) {                queue.offer(-vertical[1]);            } else {                queue.remove(vertical[1]);            }                        int current = queue.peek();            if (prev != current) {                result.add(new int[]{vertical[0], current});                prev = current;            }        }        return result;    }}