Insert Delete GetRandom O(1)

Hi Geeks! In this article, we going to see about how to insert, delete and get random elements with O(1) Time Complexity.

Design a data structure that supports all following operations in average O(1) time.

  1. insert(val): Inserts an item val to the set if not already present.
  2. remove(val): Removes an item val from the set if present.
  3. getRandom: Returns a random element from current set of elements. Each element must have the same probability of being returned.

Example:

// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();

// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);

// Returns false as 2 does not exist in the set.
randomSet.remove(2);

// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);

// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();

// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);

// 2 was already in the set, so return false.
randomSet.insert(2);

// Since 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();

Solution

public class RandomizedSet {
    
    Random rand;
    List<int> elements;
    Dictionary<int,int> map;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        rand = new Random();
        map = new Dictionary<int,int>();
        elements = new List<int>();
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public bool Insert(int val) {
        bool contain = map.ContainsKey(val);
        if(!contain)
        {
            int index = elements.Count;
            map.Add(val,index);
            elements.Add(val);
            return true;
        }
        return false;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public bool Remove(int val) {
        bool contain = map.ContainsKey(val);
        if(!contain)
            return false;
        else
        {
            int index = map[val];
            map.Remove(val);
            
            //To remove at elements
            if(index  == elements.Count-1)
                elements.RemoveAt(index);
            else
            {
                int lastElement = elements[elements.Count - 1];
                
                elements[index] = lastElement;
                map[lastElement] = index;
                
                elements.RemoveAt(elements.Count-1);
            }
            return true;
        }
    }
    
    /** Get a random element from the set. */
    public int GetRandom() {
        int randomIndex = rand.Next(0,elements.Count);
        return elements[randomIndex];
    }
}

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * bool param_1 = obj.Insert(val);
 * bool param_2 = obj.Remove(val);
 * int param_3 = obj.GetRandom();
 */

Explanation

Elements : 1 2 3 4 5 6 

  Map 

Key   Value
1       0
2       1
3       2
4       3
5       4
6       5

Remove(2)

int index = map[2]; //1
map.Remove(2);

Key   Value
1       0
3       2
4       3
5       4
6       5

int lastElement = elements[elements.Count-1]; //6
elements[index] = lastElement;
 
Elements : 1 6 3 4 5 6 

elements.RemoveAt(elements.Count-1);
Elements : 1 6 3 4 5 

map[lastElement] = index;

Key   Value
1       0
3       2
4       3
5       4
6       1

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