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

Note: Duplicate elements are allowed.

  • insert(val): Inserts an item val to the collection.
  • remove(val): Removes an item val from the collection if present.
  • 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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import java.util.Random;
public class RandomizedCollection {
    private HashMap<Integer, LinkedHashSet<Integer>> map;
    private ArrayList<Integer> nums;
    private static Random random = new Random();

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

/**
 * 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();
 */