Insert Delete GetRandom O(1)

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.

Examples:

// 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 1 is the only number in the set, getRandom always return 1.
randomSet.getRandom();

Idea:

The main design here is using a HashMap and an ArrayList to store the data.

• First save inputs pair with its location in ArrayList to the Hashmap data set.
• Second store inputs to the same location specified in HashMap.
• When removing items from the set, test if it is the last item in the list. If yes, then remove the tail of the list and remove item in Hashmap corresponding to input. If no, sway the item in the list with the last item, update the location change in the Hashmap, and then do the remove.

Solution:

public class RandomizedSet {
    public static Random rm = new Random();
    private HashMap<Integer,Integer> location;
    private ArrayList<Integer> data;

    /** Initialize your data structure here. */
    public RandomizedSet() {
        location = new HashMap<>();
        data = 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(location.containsKey(val)){
            return false;
        }
        location.put(val, data.size());
        data.add(val);
        return true;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if(!location.containsKey(val)){
            return false;
        }

        if(location.get(val) != data.size()-1){
            int loc = location.get(val);//location of val
            int tail = data.get(data.size()-1);//number of last
            //data.set(data.size()-1, data.get(loc));
            data.set(loc, tail);
            location.put(tail,loc);
        }

        location.remove(val);
        data.remove(data.size()-1);
        return true;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        return data.get(rm.nextInt(data.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();
 */