# Design a Data Structure with Insert, Delete and GetMostFrequent of O(1)

Design a data structure that allows O(1) time complexity to insert, delete and get most frequent element.

Analysis

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.

Java Solution

```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());   } }```

In the implementation above, we only add nodes to the list. We can also delete nodes that does not hold any elements.

### 1 thought on “Design a Data Structure with Insert, Delete and GetMostFrequent of O(1)”

1. I just looked at 5 problems on this site. They are not well defined – it is a little sad. For example here: What does “GetMostFrequent” actually mean? What happens if I insert an element that already exists? Does that add to some count? Does it not? A quick glance at the solution indicates the author meant some kind of linked list structure, which would indeed allow for duplicates, but that is not at all obvious from the question. Learn to formulate your questions rigorously before you decide to “help”. It is very frustrating plowing through garbage.