380. Insert Delete GetRandom O(1)

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

1. insert(val): Inserts an item val to the set if not already present.
2. remove(val): Removes an item val from the set if present.
3. 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 {
    List<Integer> set;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        set = new ArrayList();
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if(set.contains(val)) return false;
        else{
        set.add(new Integer(val));
        return true; 
        }
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if(set.contains(val)) {
            set.remove(new Integer(val));
            return true;
        }
        else return false; 
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        Random r = new Random();
        return set.get(r.nextInt(set.size()));
    }
}

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */

干？

class RandomizedSet {

    Map<Integer, Integer> map = new HashMap<>();
    List<Integer> list = new ArrayList<>();
    Random rnd = new Random();

    public boolean insert(int val) {
        if(map.containsKey(val)) {
            return false;
        }
        
        map.put(val, list.size());
        list.add(val);
        return true;
    }
    
    public boolean remove(int val) {
        if(!map.containsKey(val)) {
            return false;
        }
        // Index of value(key) in map
        int idx = map.remove(val);
        //The last element.
        int last = list.remove(list.size() - 1);
        //If equal, means the last one accidently equals to what's going to be deleted.
        
        if(val != last) {
            //If not, reset the last element to the list as index = index of what's been deleted.
            list.set(idx, last);
            //Update the index of that element.
            map.put(last, idx);
        }
        return true;
    }
    
    public int getRandom() {
        return list.get(rnd.nextInt(list.size()));
    }
}