src/main/java/com/fishercoder/solutions/_381.java

package com.fishercoder.solutions;

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

/**381. Insert Delete GetRandom O(1) - Duplicates allowed
 *
Design a data structure that supports all following operations in average O(1) time.

Note: Duplicate elements are allowed.
insert(val): Inserts an item val to the collection.
remove(val): Removes an item val from the collection if present.
getRandom: Returns a random element from current collection of elements. The probability of each element being returned is linearly related to the number of same value the collection contains.
Example:

// Init an empty collection.
_381 collection = new _381();

// Inserts 1 to the collection. Returns true as the collection did not contain 1.
collection.insert(1);

// Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1].
collection.insert(1);

// Inserts 2 to the collection, returns true. Collection now contains [1,1,2].
collection.insert(2);

// getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3.
collection.getRandom();

// Removes 1 from the collection, returns true. Collection now contains [1,2].
collection.remove(1);

// getRandom should return 1 and 2 both equally likely.
collection.getRandom();*/
public class _381 {

    Map<Integer, Integer> forwardMap;//key is the to-be-inserted number, value is its auto-incremented index
    Map<Integer, Integer> reverseMap;//the other way around
    int index;
    Random rand;

    /**
     * Initialize your data structure here.
     */
    public _381() {
        forwardMap = new HashMap();
        reverseMap = new HashMap();
        index = 0;
        rand = new Random();
    }

    /**
     * Inserts a value to the collection. Returns true if the collection did not already contain the specified element.
     */
    public boolean insert(int val) {
        boolean contains;
        if (reverseMap.containsValue(val)) {
            contains = true;
        } else {
            contains = false;
        }
        forwardMap.put(val, index);//this will overwrite the preivous key with a new index if the key already exists
        reverseMap.put(index, val);
        index++;
        return contains;
    }

    /**
     * Removes a value from the collection. Returns true if the collection contained the specified element.
     */
    public boolean remove(int val) {
        boolean contains;
        if (reverseMap.containsValue(val)) {
            contains = true;
            if (forwardMap.containsKey(val)) {
                int i = forwardMap.get(val);
                forwardMap.remove(val);
                reverseMap.remove(i);
            } else {
                //remove the entry in revserve map that has val as its value
                reverseMap.values().remove(val);
            }
        } else {
            contains = false;
        }
        return contains;
    }

    /**
     * Get a random element from the collection.
     */
    public int getRandom() {
        int randNum = rand.nextInt(index);
        while (!reverseMap.containsKey(randNum)) {
            randNum = rand.nextInt(index);
        }
        return reverseMap.get(randNum);
    }

    public static void main(String... strings) {
        _381 test = new _381();
        System.out.println(test.insert(1));
        System.out.println(test.insert(1));
        System.out.println(test.insert(2));
        System.out.println(test.getRandom());
        System.out.println(test.remove(1));
        System.out.println(test.getRandom());
    }

}