# 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

```java
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));
    }
}
```


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