Insert Delete GetRandom O(1)

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();

 class RandomizedSet {
    /** Initialize your data structure here. */
    HashSet<Integer>randomizedSet = null;
    Random random;
    public RandomizedSet() {
        randomizedSet = new HashSet<Integer>();
        random = new Random();
    }
   
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if(randomizedSet.contains(val)){
            return false;
        }
        else{
            randomizedSet.add(val);
        }
        return true;
    }
   
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
         if(randomizedSet.contains(val)){
             randomizedSet.remove(val);
            return true;
        }
        return false;
    }
   
    /** Get a random element from the set. */
    public int getRandom() {
        int rdm_number = random.nextInt(randomizedSet.size());
        int current = 0;
        Iterator<Integer> iterator = randomizedSet.iterator();
        int num;
       
        while(iterator.hasNext())
        {
            num = iterator.next();
            if(current == rdm_number)
                return num;
            current ++;
        }
        return -1;
    }
}

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();

 */

Try it on Leetcode

Here, we are going to use only Set, Iterator and Random. And in constructor we need to take care of initialization.

1) In insert() method, if number is already present then returns false. Else, insert the number to set and returns true.

2) In remove() method, if number is already present then remove the number and returns true. Else, returns false.

Above two methods can be done easily using add(), remove(), contains() methods in java.

3) In getRandom() we need to get random number at same probability of being returned.

 a) We are going to generate random number using random.nextInt(randomizedSet.size()).

 b) Creating variable current as index for iterator.

 c) If current == random_number, then return current iterator value.

In this way, we are going to get random number.

But this method is O(n). We can done O(1) using Map and List.

Below all methods are O(1) solution:

 

/** Initialize your data structure here. */
    List<Integer> list;
    Map<Integer, Integer> map;
    Random rand;
    public RandomizedSet() {
        list = new ArrayList<>();
        map = new HashMap<>();
        rand = new Random();
    }
    
    /** Inserts a value to the set. Returns true if the set did not already 
     contain the specified element. */
    public boolean insert(int val) {
        if(map.containsKey(val)){
            return false;
        }
        list.add(val);
        map.put(val, list.size() - 1);
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the     
    specified element. */
    public boolean remove(int val) {
        if(!map.containsKey(val)){
            return false;
        }
        int index = map.get(val);
        int lastElement = list.get(list.size() - 1);
        list.set(index, lastElement);
        map.put(lastElement, index);
        list.remove(list.size() - 1);
        map.remove(val);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return list.get(rand.nextInt(list.size()));
    }

Try it on Leetcode

Here to achieve O(1) in getRandom(), we are going to use Map and List. In this, map contains number and list position. Whenever any number is added or removed both list and map should be updated.

In getRadom(), we are getting random number and find a element in list using random number.

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