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();

map get, put containskey o(1)

list set, remove last one, add, get o(1)

class RandomizedSet {
    
    
    
    List<Integer> list ;
    HashMap<Integer,Integer> map;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        list = new ArrayList<>();
        
        map = new HashMap<>();
    }
    
    /** 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 valIndex = map.get(val);
        
        
        if(list.get(list.size()-1) != val){
            //swap;
            
            int last = list.get(list.size()-1);
            
            list.set(valIndex,last);
            map.put(last,valIndex);
        }
        
        map.remove(val);
        list.remove(list.size()-1);
        
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        Random random = new Random();
        return list.get(random.nextInt(list.size()));
    }
}

/**
 * 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();
 */