InsertDeleteGetRandomO1.java

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

//insert(val): Inserts an item val to the set if not already present.
//remove(val): Removes an item val from the set if present.
//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 {
    HashMap<Integer, Integer> map;
    ArrayList<Integer> values;

    /** Initialize your data structure here. */
    public RandomizedSet() {
        map = new HashMap<Integer, Integer>();
        values = new ArrayList<Integer>();
    }
    
    /** 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)) {
            map.put(val, val);
            values.add(val);
            return true;
        }
        else {
            return false;
        }
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if(map.containsKey(val)) {
            map.remove(val);
            values.remove(values.indexOf(val));
            return true;
        }
        return false;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        int random = (int)(Math.random() * values.size());
        int valueToReturn = values.get(random);
        return map.get(valueToReturn);
    }
}

/**
 * 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();
 */