class Edge {	int x;	int height;	boolean isStart; 	public Edge(int x, int height, boolean isStart) {		this.x = x;		this.height = height;		this.isStart = isStart;	}}public List
     
       getSkyline(int[][] buildings) {	List
      
        result = new ArrayList
       
        (); 	if (buildings == null || buildings.length == 0			|| buildings[0].length == 0) {		return result;	} 	List
        
          edges = new ArrayList
         
          (); 	// add all left/right edges	for (int[] building : buildings) {		Edge startEdge = new Edge(building[0], building[2], true);		edges.add(startEdge);		Edge endEdge = new Edge(building[1], building[2], false);		edges.add(endEdge);	} 	// sort edges 	Collections.sort(edges, new Comparator
          
           () { public int compare(Edge a, Edge b) { if (a.x != b.x) return Integer.compare(a.x, b.x); if (a.isStart && b.isStart) { return Integer.compare(b.height, a.height); } if (!a.isStart && !b.isStart) { return Integer.compare(a.height, b.height); } return a.isStart ? -1 : 1; } }); // process edges PriorityQueue
           
             heightHeap = new PriorityQueue
            
             (10, Collections.reverseOrder()); for (Edge edge : edges) { if (edge.isStart) { if (heightHeap.isEmpty() || edge.height > heightHeap.peek()) { result.add(new int[] { edge.x, edge.height }); } heightHeap.add(edge.height); } else { heightHeap.remove(edge.height); if(heightHeap.isEmpty()){ result.add(new int[] {edge.x, 0}); }else if(edge.height > heightHeap.peek()){ result.add(new int[]{edge.x, heightHeap.peek()}); } } } return result;}