# 20200110

Design a data structure that supports all following operations in average O(1) time.

insert(val): Inserts an item val to the set if not already present.
remove(val): Removes an item val from the set if present.
getRandom: Returns a random element from current set of elements. Each element must have the same probability of being returned.

Example:

// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();

// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);

// Returns false as 2 does not exist in the set.
randomSet.remove(2);

// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);

// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();

// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);

// 2 was already in the set, so return false.
randomSet.insert(2);

// Since 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();

class RandomizedSet {
    
    private List<Integer> list;
    private Map<Integer, Integer> map;
    private Random random;
    
    /** Initialize your data structure here. */
    public RandomizedSet() {
        list = new ArrayList<>();
        map = new HashMap<>();
        random = new Random();
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        boolean contains = map.containsKey(val);
        if (contains)
            return false;
        map.put(val, list.size());
        list.add(val);
        
        return true;
    }
    
    // Step1: 1, 2, 3, 4
    // Step2: 1, 4, 3, 4
    // Step3: 1, 4, 3
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        boolean contains = map.containsKey(val);
        if (!contains) 
            return false;
        int position = map.get(val);
        if (position < list.size() - 1) {
            int lastNum = list.get(list.size() - 1);
            list.set(position, lastNum);
            map.put(lastNum, position);
        }
        
        map.remove(val);
        list.remove(list.size() - 1);
        
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return list.get(random.get(list.size()));
    }
}