Question

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();

Solution

class RandomizedSet {
    Map<Integer, Integer> map;
    ArrayList<Integer> array;
    Random random;
    int size;

    /** Initialize your data structure here. */
    public RandomizedSet() {
        this.map = new HashMap<Integer, Integer>();
        this.array = new ArrayList<Integer>();
        this.random = new Random(System.currentTimeMillis());
        this.size = 0;
    }

    /** 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, size++);
            array.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)) {
            int idx = map.remove(val);
            if(idx == --size) array.remove(size);
            else {
                int lastVal = array.remove(size);
                array.set(idx, lastVal);
                map.replace(lastVal, idx);
            }
            return true;
        } else return false;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        int idx = random.nextInt(size);
        return array.get(idx);
    }
}

/**
 * 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();
 */