Design a Data Structure with Insert & Delete & GetMostFrequent of O(1) - LRU


https://www.programcreek.com/2016/08/design-a-data-structure-with-insert-delete-getmostfrequent-of-o1/
Design a data structure that allows O(1) time complexity to insert, delete and get most frequent element.

At first, a hash map seems to be good for insertion and deletion. But how to make getMostFrequent O(1)? Regular sorting algorithm takes nlogn, so we can not use that. As a result we can use a linked list to track the maximum frequency.


import java.util.*;
 
class Node {
 int value;
 Node prev;
 Node next;
 HashSet<Integer> set;
 
 public Node(int v){
  value = v;
  set = new HashSet<Integer>();
 }
 
 public String toString(){
  return value + ":" + set.toString();
 }
}
 
public class FrequentCollection { 
 
 HashMap<Integer, Node> map;
 Node head, tail;
 
 /** Initialize your data structure here. */
 public FrequentCollection() {
  map = new HashMap<Integer, Node>();
 }
 
 /**
  * Inserts a value to the collection. 
  */
 public void insert(int val) {
  if(map.containsKey(val)){
   Node n = map.get(val);
   n.set.remove(val);
 
   if(n.next!=null){
    n.next.set.add(val); // next + 1
    map.put(val, n.next);
   }else{
    Node t = new Node(n.value+1);
    t.set.add(val);
    n.next = t;
    t.prev = n;
    map.put(val, t);
   }
 
   //update head
   if(head.next!=null)
    head = head.next;
  }else{
   if(tail==null||head==null){
    Node n = new Node(1);
    n.set.add(val);
    map.put(val, n);
 
    head = n;
    tail = n;
    return;
   }
 
   if(tail.value>1){
    Node n = new Node(1);
    n.set.add(val);
    map.put(val, n);
    tail.prev = n;
    n.next = tail;
    tail = n;
   }else{
    tail.set.add(val);
    map.put(val, tail);
   }
 
  }
 
 
 }
 
 /**
  * Removes a value from the collection. 
  */
 public void remove(int val) {
  Node n = map.get(val);
  n.set.remove(val);
 
  if(n.value==1){
   map.remove(val);
  }else{
   n.prev.set.add(val);
   map.put(val, n.prev);
  }
 
 
  while(head!=null && head.set.size()==0){
   head = head.prev;
  }
 
 
 }
 
 /** Get the most frequent element from the collection. */
 public int getMostFrequent() {
  if(head==null)
   return -1;
  else
   return head.set.iterator().next();
 }
 
 public static void main(String[] args) {
  // TODO Auto-generated method stub
  FrequentCollection fc = new FrequentCollection();
  fc.insert(1);
  fc.insert(2);
  fc.insert(3);
  fc.insert(2);
  fc.insert(3);
  fc.insert(3);
  fc.insert(2);
  fc.insert(2);
 
  System.out.println(fc.getMostFrequent());
  fc.remove(2);
  fc.remove(2);
  System.out.println(fc.getMostFrequent());
 
 }
}




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