Insert Delete GetRandom O(1)

Description

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

Hint

Train of Thought

I got a similar question for my phone interview. The difference is that the duplicated number is allowed. So, think that is a follow-up of this question. How do you modify your code to allow duplicated number?

Code

public class RandomizedSet {
ArrayList<Integer> nums;
HashMap<Integer, Integer> locs;
java.util.Random rand = new java.util.Random();
/** Initialize your data structure here. */
public RandomizedSet() {
    nums = new ArrayList<Integer>();
    locs = new HashMap<Integer, Integer>();
}

/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
public boolean insert(int val) {
    boolean contain = locs.containsKey(val);
    if ( contain ) return false;
    locs.put( val, nums.size());
    nums.add(val);
    return true;
}

/** Removes a value from the set. Returns true if the set contained the specified element. */
public boolean remove(int val) {
    boolean contain = locs.containsKey(val);
    if ( ! contain ) return false;
    int loc = locs.get(val);
    if (loc < nums.size() - 1 ) { // not the last one than swap the last one with this val
        int lastone = nums.get(nums.size() - 1 );
        nums.set( loc , lastone );
        locs.put(lastone, loc);
    }
    locs.remove(val);
    nums.remove(nums.size() - 1);
    return true;
}

/** Get a random element from the set. */
public int getRandom() {
    return nums.get( rand.nextInt(nums.size()) );
}

}

Complexity