0381. Insert Delete GetRandom O(solutions/1) - Duplicates allowed

https://leetcode.com/problems/insert-delete-getrandom-o1-duplicates-allowed

Description

Implement the RandomizedCollection class:

• RandomizedCollection() Initializes the RandomizedCollection object.

• bool insert(int val) Inserts an item val into the multiset if not present. Returns true if the item was not present, false otherwise.

• bool remove(int val) Removes an item val from the multiset if present. Returns true if the item was present, false otherwise. Note that if val has multiple occurrences in the multiset, we only remove one of them.

• int getRandom() Returns a random element from the current multiset of elements (it's guaranteed that at least one element exists when this method is called). The probability of each element being returned is linearly related to the number of same values the multiset contains.

You must implement the functions of the class such that each function works in average O(1) time complexity.

Example 1:

**Input**
["RandomizedCollection", "insert", "insert", "insert", "getRandom", "remove", "getRandom"]
[[], [1], [1], [2], [], [1], []]
**Output**
[null, true, false, true, 2, true, 1]
**Explanation**
RandomizedCollection randomizedCollection = new RandomizedCollection();
randomizedCollection.insert(1);   // return True. Inserts 1 to the collection. Returns true as the collection did not contain 1.
randomizedCollection.insert(1);   // return False. Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1].
randomizedCollection.insert(2);   // return True. Inserts 2 to the collection, returns true. Collection now contains [1,1,2].
randomizedCollection.getRandom(); // getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3.
randomizedCollection.remove(1);   // return True. Removes 1 from the collection, returns true. Collection now contains [1,2].
randomizedCollection.getRandom(); // getRandom should return 1 and 2 both equally likely.

Constraints:

• -231 <= val <= 231 - 1

• At most 2 * 105 calls will be made to insert, remove, and getRandom.

• There will be at least one element in the data structure when getRandom is called.

ac

class RandomizedCollection {
    Random random;
    List<Integer> list;
    Map<Integer, Set<Integer>> map;
    int index;
    /** Initialize your data structure here. */
    public RandomizedCollection() {
        random = new Random();
        list = new ArrayList<>();
        map = new HashMap<>();
        index = 0;
    }

    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        boolean res = false;
        if (!map.containsKey(val)) {
            map.put(val, new HashSet<>());
            res = true;
        }
        list.add(val);
        map.get(val).add(index);
        index++;

        return res;
    }

    /** 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;

        // get target, remove in map-set
        int targetIndex = map.get(val).iterator().next();
        map.get(val).remove(targetIndex);
        if (map.get(val).isEmpty()) map.remove(val); // remove empty set
        // swap, remove in list
        int lastIndex = index - 1;
        int lastVal = list.get(lastIndex);
        list.set(targetIndex, lastVal);
        list.remove(lastIndex);
        index--;
        // update index in map-set
        if (lastIndex != targetIndex) {
            map.get(lastVal).remove(lastIndex);
            map.get(lastVal).add(targetIndex);
        }

        return true;
    }

    /** Get a random element from the collection. */
    public int getRandom() {
        return list.get(random.nextInt(index));
    }
}