Insert Delete GetRandom O(1)

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 1 is the only number in the set, getRandom always return 1.
randomSet.getRandom();

Tips:

for O(1) insert and remove we have to use something utilizing hashcode, like hashmap or hashset.

And for constant time getRandom, we will need something that has random access like array or arraylist.

We can combine them together, using a hashmap to store a map from value to their index in the arraylist.

Code:

public class RandomizedSet {
    Map<Integer, Integer> map;
    List<Integer> vals;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        map = new HashMap<>();
        vals = new  ArrayList<>();         
    }

    /** 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;
        }
        map.put(val, vals.size());
        vals.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 (!map.containsKey(val)) {
            return false;
        }
        int last = vals.get(vals.size() - 1);
        vals.set(map.get(val), last);
        map.put(last, map.get(val));
        map.remove(val);
        vals.remove(vals.size() - 1);
        return true;        
    }

    /** Get a random element from the set. */
    public int getRandom() {
        Random rand = new Random();
        int r = rand.nextInt(vals.size());
        return vals.get(r);        
    }
}

/**
 * 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();
 */