https://leetcode.com/problems/the-skyline-problem/

class Solution {
    class Point {
        int x, y,s;
        Point(int x, int y, int s) {
            this.x = x;
            this.y = y;
            this.s = s;
        }
//         @Override
//         public boolean equals(Object o) { 
//             Point c = (Point) o;         
//         return Integer.compare(y, c.y) == 0; 
        // } 
    }

    Queue<Integer> pq = new PriorityQueue<>((a,b) -> b - a); // sort by y
    List<Point> pointlist = new ArrayList<>();

    public List<int[]> getSkyline(int[][] buildings) {
         List<int[]> list = new ArrayList<>();
        if (buildings == null || buildings.length == 0|| buildings[0] == null || buildings[0].length == 0) return list;
        //add point (x,y,s) to pointlist start s:0 end s:1
        for (int[] building: buildings) {
            Point p1 = new Point(building[0],building[2], 0);
            Point p2 = new Point(building[1],building[2], 1);
            pointlist.add(p1);
            pointlist.add(p2);

        }
        Collections.sort(pointlist,(a,b)-> a.x-b.x);// sort by x

        int prevh = 0;
        pq.offer(0);//*!
        int curx = pointlist.get(0).x;// initial x
        int index = 0; // index++ only when at same x, otherwise need to generate ans
        while (index < pointlist.size()) {
            Point p = pointlist.get(index);
            if (p.x == curx) { 
                if (p.s == 0) {
                    pq.offer(p.y);
                } else {
                    pq.remove(p.y);
                }
                index++;
            } else { //move to next x then generate ans
                int curh = pq.peek();
                //System.out.println(cur);
                if (curh != prevh) {
                    int[] ans = new int[] {curx, curh};
                    prevh = curh; 
                    list.add(ans);
                }
                curx = p.x;

            }


        }
        //add last one
        int[] ans = new int[] {curx, 0};
        list.add(ans);
        return list;
    }
}