public int[][] reconstructQueue(int[][] people) {
        //pick up the tallest guy first
        //when insert the next tall guy, just need to insert him into kth position
        //repeat until all people are inserted into list
        Arrays.sort(people,new Comparator<int[]>(){
           @Override
           public int compare(int[] o1, int[] o2){
               return o1[0]!=o2[0]?-o1[0]+o2[0]:o1[1]-o2[1];
           }
        });
        List<int[]> res = new LinkedList<>();
        for(int[] cur : people){
            res.add(cur[1],cur);       
        }
        return res.toArray(new int[people.length][]);
    }

Java 8
public int[][] reconstructQueue(int[][] people) {
        Arrays.sort(people, (a, b) -> a[0] != b[0] ? b[0] - a[0] : a[1] - b[1]);
        List<int[]> res = new LinkedList<>();
        for(int[] e : people){
            res.add(e[1], e);
        }
        return res.toArray(new int[people.length][]);
    }

https://leetcode.com/problems/queue-reconstruction-by-height/

https://leetcode.com/problems/merge-intervals/discuss/

Priority Queue and maxheap

import java.util.Comparator;
import java.util.PriorityQueue;

public class PriorityQueueTest {

    static class PQsort implements Comparator<Integer> {

        public int compare(Integer one, Integer two) {
            return two - one;
        }
    }

    public static void main(String[] args) {
        int[] ia = { 1, 10, 5, 3, 4, 7, 6, 9, 8 };
        PriorityQueue<Integer> pq1 = new PriorityQueue<Integer>();

        // use offer() method to add elements to the PriorityQueue pq1
        for (int x : ia) {
            pq1.offer(x);
        }

        System.out.println("pq1: " + pq1);

        PQsort pqs = new PQsort();
        PriorityQueue<Integer> pq2 = new PriorityQueue<Integer>(10, pqs);
        // In this particular case, we can simply use Collections.reverseOrder()
        // instead of self-defined comparator
        for (int x : ia) {
            pq2.offer(x);
        }

        System.out.println("pq2: " + pq2);

        // print size
        System.out.println("size: " + pq2.size());
        // return highest priority element in the queue without removing it
        System.out.println("peek: " + pq2.peek());
        // print size
        System.out.println("size: " + pq2.size());
        // return highest priority element and removes it from the queue
        System.out.println("poll: " + pq2.poll());
        // print size
        System.out.println("size: " + pq2.size());

        System.out.print("pq2: " + pq2);

    }
}

Output:

pq1: [1, 3, 5, 8, 4, 7, 6, 10, 9]
pq2: [10, 9, 7, 8, 3, 5, 6, 1, 4]
size: 9
peek: 10
size: 9
poll: 10
size: 8
pq2: [9, 8, 7, 4, 3, 5, 6, 1]

http://www.programcreek.com/2009/02/using-the-priorityqueue-class-example/

TreeMap:

import java.util.*;
public class TreeMapDemo {

   public static void main(String args[]) {
      // Create a hash map
      TreeMap tm = new TreeMap();

      // Put elements to the map
      tm.put("Zara", new Double(3434.34));
      tm.put("Mahnaz", new Double(123.22));
      tm.put("Ayan", new Double(1378.00));
      tm.put("Daisy", new Double(99.22));
      tm.put("Qadir", new Double(-19.08));

      // Get a set of the entries
      Set set = tm.entrySet();

      // Get an iterator
      Iterator i = set.iterator();

      // Display elements
      while(i.hasNext()) {
         Map.Entry me = (Map.Entry)i.next();
         System.out.print(me.getKey() + ": ");
         System.out.println(me.getValue());
      }
      System.out.println();

      // Deposit 1000 into Zara's account
      double balance = ((Double)tm.get("Zara")).doubleValue();
      tm.put("Zara", new Double(balance + 1000));
      System.out.println("Zara's new balance: " + tm.get("Zara"));
   }
}

Output:

Ayan: 1378.0
Daisy: 99.22
Mahnaz: 123.22
Qadir: -19.08
Zara: 3434.34

Zara's new balance: 4434.34



java.util.TreeMap.pollLastEntry() and java.util.TreeMap.pollFirstEntry()

package com.tutorialspoint;

import java.util.*;

public class TreeMapDemo {
   public static void main(String[] args) {
   // creating tree map 
   TreeMap<Integer, String> treemap = new TreeMap<Integer, String>();

   // populating tree map
   treemap.put(2, "two");
   treemap.put(1, "one");
   treemap.put(3, "three");
   treemap.put(6, "six");
   treemap.put(5, "five");   

   // polling last entry
   System.out.println("Value before poll: "+ treemap);            
   System.out.println("Value returned: "+ treemap.pollLastEntry()); 
   System.out.println("Value returned: "+ treemap.pollFirstEntry());     
   System.out.println("Value after poll: "+ treemap);
   }    
}

Output:
Value before poll: {1=one, 2=two, 3=three, 5=five, 6=six}
Value returned: 6=six
Value returned: 1=one
Value after poll: {1=one, 2=two, 3=three, 5=five}

https://www.tutorialspoint.com/java/java_treemap_class.htm

https://www.tutorialspoint.com/java/util/treemap_polllastentry.htm