380 Insert Delete GetRandom O(1)

Problem

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();

Solutions:

public class RandomizedSet {
    private HashMap<Integer, Integer> numToIndex;
    private HashMap<Integer, Integer> indexToNum;
    private int size;
    private Random r;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        numToIndex = new HashMap<Integer, Integer>();
        indexToNum = new HashMap<Integer, Integer>();
        size = 0;
        r = 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) {
        if (!numToIndex.containsKey(val)) {
            indexToNum.put(size, val);
            numToIndex.put(val, size);
            size ++;
            return true;
        }
        return false;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (numToIndex.containsKey(val)) {
            int index = numToIndex.get(val);
            int lastVal = indexToNum.get(size - 1);
            indexToNum.put(index, lastVal);
            indexToNum.remove(size - 1);
            numToIndex.put(lastVal, index);
            numToIndex.remove(val);
            size --;
            return true;
        }
        return false;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        if (size == 0) {
            return -1;
        }
        int index = r.nextInt(size);
        int val = indexToNum.get(index);
        return val;
    }
}

/**
 * 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();
 */