src/main/java/com/fishercoder/solutions/_218.java

package com.fishercoder.solutions;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.TreeMap;

public class _218 {

    public static class Solution1 {

        class BuildingPoint implements Comparable<BuildingPoint> {
            int x;
            boolean isStart;
            int h;

            public BuildingPoint(int x, boolean isStart, int h) {
                this.x = x;
                this.h = h;
                this.isStart = isStart;
            }

            @Override
            public int compareTo(BuildingPoint o) {
                if (this.x != o.x) {
                    return this.x - o.x;
                } else {
                    if (this.isStart && o.isStart) {
                        return o.h - this.h;
                    } else if (this.isStart && !o.isStart) {
                        return -this.h - o.h;
                    } else if (!this.isStart && !o.isStart) {
                        return this.h - o.h;
                    } else {
                        return this.h + o.h;
                    }
                }
            }
        }

        public List<List<Integer>> getSkyline(int[][] buildings) {
            BuildingPoint[] bps = new BuildingPoint[buildings.length * 2];
            int index = 0;
            for (int[] building : buildings) {
                BuildingPoint bp1 = new BuildingPoint(building[0], true, building[2]);
                BuildingPoint bp2 = new BuildingPoint(building[1], false, building[2]);
                bps[index++] = bp1;
                bps[index++] = bp2;
            }

            //this is one key step:
            Arrays.sort(bps);

            List<List<Integer>> result = new ArrayList();
            TreeMap<Integer, Integer> treeMap = new TreeMap();
            treeMap.put(0, 1);
            int prevMaxH = 0;
            for (BuildingPoint bp : bps) {
                //if it's a starting point, we'll add it into the final result
                if (bp.isStart) {
                    if (treeMap.containsKey(bp.h)) {
                        treeMap.put(bp.h, treeMap.get(bp.h) + 1);
                    } else {
                        treeMap.put(bp.h, 1);
                    }
                } else if (!bp.isStart) {
                    //if it's an ending point, we'll decrement/remove this entry
                    if (treeMap.containsKey(bp.h) && treeMap.get(bp.h) > 1) {
                        treeMap.put(bp.h, treeMap.get(bp.h) - 1);
                    } else {
                        treeMap.remove(bp.h);
                    }
                }

                int currMaxH = treeMap.lastKey();
                if (currMaxH != prevMaxH) {
                    result.add(Arrays.asList(bp.x, currMaxH));
                    prevMaxH = currMaxH;
                }

            }

            return result;
        }
    }
}