src/main/java/com/fishercoder/solutions/_380.java

package com.fishercoder.solutions;

import java.util.HashMap;
import java.util.Map;
import java.util.Random;

/**
 * 380. Insert Delete GetRandom O(1)
 * Design a data structure that supports all following operations in average O(1) time.
 * <p>
 * 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:
 * <p>
 * // Init an empty set.
 * RandomizedSet randomSet = new RandomizedSet();
 * <p>
 * // Inserts 1 to the set. Returns true as 1 was inserted successfully.
 * randomSet.insert(1);
 * <p>
 * // Returns false as 2 does not exist in the set.
 * randomSet.remove(2);
 * <p>
 * // Inserts 2 to the set, returns true. Set now contains [1,2].
 * randomSet.insert(2);
 * <p>
 * // getRandom should return either 1 or 2 randomly.
 * randomSet.getRandom();
 * <p>
 * // Removes 1 from the set, returns true. Set now contains [2].
 * randomSet.remove(1);
 * <p>
 * // 2 was already in the set, so return false.
 * randomSet.insert(2);
 * <p>
 * // Since 2 is the only number in the set, getRandom always return 2.
 * randomSet.getRandom();
 */

public class _380 {
    /**
     * Your _380 object will be instantiated and called as such:
     * _380 obj = new _380();
     * boolean param_1 = obj.insert(val);
     * boolean param_2 = obj.delete(val);
     * int param_3 = obj.getRandom();
     */
//TODO: this is not ideal, optimize it.
    public static class RandomizedSet {

        Map<Integer, Integer> forwardMap;//key is auto increment index, value if the inserted val
        Map<Integer, Integer> reverseMap;//the other way around
        int index;
        Random random;

        /**
         * Initialize your data structure here.
         */
        public RandomizedSet() {
            forwardMap = new HashMap();
            reverseMap = new HashMap();
            index = 0;
            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) {
            if (forwardMap.containsValue(val)) {
                return false;
            } else {
                forwardMap.put(index, val);
                reverseMap.put(val, index++);
                return true;
            }
        }

        /**
         * Deletes a value from the set. Returns true if the set contained the specified element.
         */
        public boolean remove(int val) {
            if (forwardMap.containsValue(val)) {
                int key = reverseMap.get(val);
                reverseMap.remove(val);
                forwardMap.remove(key);
                return true;
            } else {
                return false;
            }
        }

        /**
         * Get a random element from the set.
         */
        public int getRandom() {
            int max = forwardMap.size();
            if (max == 1) {
                return forwardMap.get(index - 1);
            }
            int randomNum = random.nextInt(max);
            while (!forwardMap.containsKey(randomNum)) {
                randomNum = random.nextInt(max);
            }
            return forwardMap.get(randomNum);
        }
    }
}