JavaAlgorithms/Others/SkylineProblem.java

import java.util.ArrayList;
import java.util.Iterator;
import java.util.Scanner;

public class SkylineProblem {
	Building[] building;
	int count;
	
	public void run() {
		Scanner sc = new Scanner(System.in);
		
		int num = sc.nextInt();
		this.building = new Building[num];
		
		for(int i = 0; i < num; i++) {
			String input = sc.next();
			String[] data = input.split(",");
			this.add(Integer.parseInt(data[0]), Integer.parseInt(data[1]), Integer.parseInt(data[2]));
		}
		this.print(this.findSkyline(0, num - 1));
		
		sc.close();
	}
	
	public void add(int left, int height, int right) {
		building[count++] = new Building(left, height, right);
	}
	
	public void print(ArrayList<Skyline> skyline) {
		Iterator<Skyline> it = skyline.iterator();
		
		while(it.hasNext()) {
			Skyline temp = it.next();
			System.out.print(temp.coordinates + "," + temp.height);
			if(it.hasNext()) {
				System.out.print(",");
			}
		}
		
	}
	
	public ArrayList<Skyline> findSkyline(int start, int end) {
		if(start == end) {
			ArrayList<Skyline> list = new ArrayList<>();
			list.add(new Skyline(building[start].left, building[start].height));
			list.add(new Skyline(building[end].right, 0));
			
			return list;
		}
		
		int mid = (start + end) / 2;
		
		ArrayList<Skyline> sky1 = this.findSkyline(start, mid);
		ArrayList<Skyline> sky2 = this.findSkyline(mid + 1, end);
		
		return this.mergeSkyline(sky1, sky2);
	}
	
	public ArrayList<Skyline> mergeSkyline(ArrayList<Skyline> sky1, ArrayList<Skyline> sky2) {
		int currentH1 = 0, currentH2 = 0;
		ArrayList<Skyline> skyline = new ArrayList<>();
		int maxH = 0;
		
		while(!sky1.isEmpty() && !sky2.isEmpty()) {
			if(sky1.get(0).coordinates < sky2.get(0).coordinates) {
				int currentX = sky1.get(0).coordinates;
				currentH1 = sky1.get(0).height;
				
				if(currentH1 < currentH2) {
					sky1.remove(0);
					if(maxH != currentH2) skyline.add(new Skyline(currentX, currentH2));
				} else {
					maxH = currentH1;
					sky1.remove(0);
					skyline.add(new Skyline(currentX, currentH1));
				}
			} else {
				int currentX = sky2.get(0).coordinates;
				currentH2 = sky2.get(0).height;
				
				if(currentH2 < currentH1) {
					sky2.remove(0);
					if(maxH != currentH1) skyline.add(new Skyline(currentX, currentH1));
				} else {
					maxH = currentH2;
					sky2.remove(0);
					skyline.add(new Skyline(currentX, currentH2));
				}
			}
		}

		while(!sky1.isEmpty()) {
			skyline.add(sky1.get(0));
			sky1.remove(0);
		}
		
		while(!sky2.isEmpty()) {
			skyline.add(sky2.get(0));
			sky2.remove(0);
		}
		
		return skyline;
	}
	
	public class Skyline {
		public int coordinates;
		public int height;
		
		public Skyline(int coordinates, int height) {
			this.coordinates = coordinates;
			this.height = height;
		}
	}
	
	public class Building {
		public int left;
		public int height;
		public int right;
		
		public Building(int left, int height, int right) {
			this.left = left;
			this.height = height;
			this.right = right;
		}
	}
	
	public static void main(String[] args) {
		SkylineProblem skylineProblem = new SkylineProblem();
		skylineProblem.run();
	}
}