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();

Solution:

Use a HashMap and an ArrayList.

ArrayList is to store the elements and the HashMap is to mapping the position of the element.

To insert an element, simply check if its already in the map. If not, we append it to the ArrayList and update the HashMap.

To remove an element, we use HashMap to find its location, swap with the end of the ArrayList, and remove it. Don't forget to update the mapping of the end element which swapped location.



Code:

public class RandomizedSet {

    ArrayList<Integer> list;
    HashMap<Integer, Integer> map;
    java.util.Random rand = new java.util.Random();
    /** Initialize your data structure here. */
    public RandomizedSet() {
        list = new ArrayList<>();
        map = new HashMap<>();
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if (map.containsKey(val)) {
            return false;
        }
        list.add(val);
        map.put(val, list.size() - 1);
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (!map.containsKey(val)) {
            return false;
        }
        int key = list.get(list.size() - 1);
        int index = map.get(val);
        Collections.swap(list, index, list.size() - 1);
        map.put(key, index);
        map.remove(list.get(list.size() - 1));
        list.remove(list.size() - 1);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return list.get(rand.nextInt(list.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();
 */