381. Insert Delete GetRandom O(1)

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();

较之于380. Insert Delete GetRandom O(1)允许重复添加数，代码上稍微有一点变化，代码如下：

public class RandomizedCollection {    ArrayList<Integer> nums;    HashMap<Integer, Set<Integer>> map;    java.util.Random rand = new java.util.Random();    /** Initialize your data structure here. */    public RandomizedCollection() {        nums = new ArrayList<Integer>();        map = new HashMap<Integer, Set<Integer>>();    }        /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */    public boolean insert(int val) {        boolean contain = map.containsKey(val);        if (!contain) {            map.put(val, new HashSet<Integer>());        }        map.get(val).add(nums.size());        nums.add(val);        return !contain;    }        /** Removes a value from the collection. Returns true if the collection contained the specified element. */    public boolean remove(int val) {        boolean contain = map.containsKey(val);        if (!contain) {            return false;        }        int loc = map.get(val).iterator().next();        map.get(val).remove(loc);        if (loc < nums.size() - 1) {            int lastone = nums.get(nums.size() - 1);            map.get(lastone).remove(nums.size() - 1);            map.get(lastone).add(loc);            nums.set(loc, lastone);        }        nums.remove(nums.size() - 1);        if (map.get(val).isEmpty()) {            map.remove(val);        }        return true;    }        /** Get a random element from the collection. */    public int getRandom() {        return nums.get(rand.nextInt(nums.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(); */