381. Insert Delete GetRandom O(1) - Duplicates allowed

381. Insert Delete GetRandom O(1) - Duplicates allowed

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

Note: Duplicate elements are allowed.

1. insert(val): Inserts an item val to the collection.
2. remove(val): Removes an item val from the collection if present.
3. getRandom: Returns a random element from current collection of elements. The probability of each element being returned is linearly related to the number of same value the collection contains.

Example:

// Init an empty collection.
RandomizedCollection collection = new RandomizedCollection();

// Inserts 1 to the collection. Returns true as the collection did not contain 1.
collection.insert(1);

// Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1].
collection.insert(1);

// Inserts 2 to the collection, returns true. Collection now contains [1,1,2].
collection.insert(2);

// getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3.
collection.getRandom();

// Removes 1 from the collection, returns true. Collection now contains [1,2].
collection.remove(1);

// getRandom should return 1 and 2 both equally likely.
collection.getRandom();

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class RandomizedCollection {
    
    private static Random random = new Random();
    private Map<Integer, Set<Integer>> map;
    private List<Integer> list;
    private int index;

    /** Initialize your data structure here. */
    public RandomizedCollection() {
        index = 0;
        list = new ArrayList<>();
        map = new HashMap<>();
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        boolean rt = false;
        if (!map.containsKey(val)) {
            map.put(val, new HashSet<Integer>());
            rt = true;
        }
        map.get(val).add(index);
        
        if (index == list.size()) {
            list.add(val);
        }else {
            list.set(index, val);
        }
        
        index++;
        
        return rt;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        if (!map.containsKey(val) || map.get(val).isEmpty()) return false;
        
        int pos = map.get(val).iterator().next();
        map.get(val).remove(pos);
        int endValue = list.get(index - 1);
        map.get(endValue).add(pos);
        map.get(endValue).remove(index - 1);
        list.set(pos, endValue);
        index--;
            
        return true;
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        return list.get(random.nextInt(index));
    }
}

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