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

https://leetcode.com/problems/insert-delete-getrandom-o1-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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Related problems
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class RandomizedCollection {
// Actual storage of val, return entry from a random index
List<Integer> list;
// Map the value to indices of value in List
Map<Integer, Set<Integer>> map;
// Random number generator, create it once for the class
Random random;
/** Initialize your data structure here. */
public RandomizedCollection() {
list = new ArrayList<Integer>();
map = new HashMap<Integer, Set<Integer>>();
random = 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) {
// Initialize empty set
map.putIfAbsent(val, new HashSet<Integer>());
// Add last index for the new val
map.get(val).add(list.size());
list.add(val);
// True if val is added for first time
return map.get(val).size() == 1;
}
/** 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).size() == 0) {
return false;
}
// Fetch index of val to remove
int indexToRemove = map.get(val).iterator().next();
// Fetch last value from list
int last = list.get(list.size() - 1);
// Add the new position to last value
map.get(last).add(indexToRemove);
// *** Corner case *** Move the last value to firstIndex only if not duplicate
if (last != val) {
// Copy last value to position of value to be removed
list.set(indexToRemove, last);
map.get(val).remove(indexToRemove);
}
// Remove the last value from end of list
map.get(last).remove(list.size() - 1);
list.remove(list.size() - 1);
return true;
}
/** Get a random element from the collection. */
public int getRandom() {
return list.get(random.nextInt(list.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();
*/