381.insert-delete-get-random-o-1-duplicates-allowed.java

/*
 * @lc app=leetcode id=381 lang=java
 *
 * [381] Insert Delete GetRandom O(1) - Duplicates allowed
 */

// @lc code=start
class RandomizedCollection {
    Map<Integer, Set<Integer>> map;
    List<Integer> list;
    Random rand = new Random();
    
    /** Initialize your data structure here. */
    public RandomizedCollection() {
        map = new HashMap<>();
        list = new ArrayList<>();
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        if (!map.containsKey(val)) {
            map.put(val, new HashSet<Integer>());
        }
        map.get(val).add(list.size());
        list.add(val);
        
        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;
        }

        int lastElement = list.get(list.size() - 1);
        int index = map.get(val).iterator().next();
        map.get(val).remove(index);
        // replace and add
        list.set(index, lastElement);
        map.get(lastElement).add(index);
        //delete
        map.get(lastElement).remove(list.size() - 1);
        list.remove(list.size() - 1);

        return true;
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        return list.get(rand.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();
 */
// @lc code=end