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();--------------------------
class RandomizedCollection { private static Random random = new Random(); private Map<Integer, Set<Integer>> map; private List<Integer> list; private int index; /** Initialize your data structure here. */ public RandomizedCollection() { index = 0; list = new ArrayList<>(); map = new HashMap<>(); } /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */ public boolean insert(int val) { boolean rt = false; if (!map.containsKey(val)) { map.put(val, new HashSet<Integer>()); rt = true; } map.get(val).add(index); if (index == list.size()) { list.add(val); }else { list.set(index, val); } index++; return rt; } /** 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).isEmpty()) return false; int pos = map.get(val).iterator().next(); map.get(val).remove(pos); int endValue = list.get(index - 1); map.get(endValue).add(pos); map.get(endValue).remove(index - 1); list.set(pos, endValue); index--; return true; } /** Get a random element from the collection. */ public int getRandom() { return list.get(random.nextInt(index)); } } /** * 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(); */
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