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();

380. Insert Delete GetRandom O(1) - Medium 的 follow-up

還是用hashmap和arraylist，hashmap中存<val, [val indices]>, arraylist中存 (val, index in hashmap)，即 (val, val在map.get(val)中對應的下標)

insert: 直接加在hash map對應的list末尾和arraylist的末尾

delete: 和最後一個元素交換，再刪除最後一個元素。注意如果map.get(val)只有一個元素的時候，要把map中對應的val刪除

class RandomizedCollection {
    Map<Integer, List<Integer>> map;
    List<int[]> list;
    Random r;

    /** Initialize your data structure here. */
    public RandomizedCollection() {
        map = new HashMap<>();
        list = new ArrayList<>();
        r = new Random();
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        if(map.containsKey(val)) {
            int idx = map.get(val).size() - 1;
            list.add(new int[] {val, idx + 1});
            map.get(val).add(list.size() - 1);
            return false;
        }
        else {
            list.add(new int[] {val, 0});
            map.put(val, new ArrayList<>());
            map.get(val).add(list.size() - 1);
            return true;
        }
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        if(!map.containsKey(val)) {
            return false;
        }
        List<Integer> idxs = map.get(val);
        int idx = idxs.get(idxs.size() - 1);
        if(idxs.size() == 1) {
            int[] last = list.get(list.size() - 1);
            list.set(idx, last);
            list.remove(list.size() - 1);
            List<Integer> last_idxs = map.get(last[0]);
            last_idxs.set(last[1], idx);
            map.remove(val);
            return true;
        }
        if(idx < list.size() - 1) {
            int[] last = list.get(list.size() - 1);
            list.set(idx, last);
            List<Integer> last_idxs = map.get(last[0]);
            last_idxs.set(last[1], idx);
        }
        list.remove(list.size() - 1);
        List<Integer> tmp = map.get(val);
        tmp.remove(tmp.size() - 1);
        
        return true;
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        return list.get(r.nextInt(list.size()))[0];
    }
}

/**
 * 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();
 */