[LeetCode] 381. Insert Delete GetRandom

Problem

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();

Solution

class RandomizedCollection {

    List list;
    Map> map;
    /** Initialize your data structure here. */
    public RandomizedCollection() {
        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 notContain = !map.containsKey(val);
        if (notContain) map.put(val, new HashSet());
        list.add(val);
        map.get(val).add(list.size()-1);
        return notContain;
    }
    
    /** 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;
        if (!map.get(val).contains(list.size()-1)) {
            int index = map.get(val).iterator().next();
            int last = list.get(list.size()-1);
            map.get(last).remove(list.size()-1);
            map.get(last).add(index);
            map.get(val).remove(index);
            map.get(val).add(list.size()-1);
            list.set(index, last);
            //then the element would be 'last' in both index and list.size()-1
        }
        map.get(val).remove(list.size()-1);
        if (map.get(val).isEmpty()) map.remove(val);
        list.remove(list.size()-1);
        return true;
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        int min = 0, max = list.size();
        int index = (int)(Math.random()*(max-min)+min);
        return list.get(index);
    }
}