Question

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();

Solution

  • Method 1: HashMap + ArrayList 
    class RandomizedSet {
    private Map<Integer, Integer> map;
    private List<Integer> nums;
    private Random random;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        this.map = new HashMap<>();
        this.nums = new ArrayList<>();
        this.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(map.containsKey(val)) return false;
        nums.add(val);
        map.put(val, nums.size() - 1);
        return true;
    }

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

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

Amazon session

class RandomizedSet {
    private Random random;
    private Map<Integer, Integer> map;
    private List<Integer> list;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        this.random = new Random();
        this.map = new HashMap<>(); //key: given key, value: index in the array list
        this.list = 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(map.containsKey(val)) return false;
        int size = list.size();
        map.put(val, size);
        list.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(!map.containsKey(val)) return false;
        int last = list.get(list.size() - 1);   //last value
        int index = map.get(val);
        if(index != list.size() - 1){
            list.set(index, last);
            map.put(last, index);
        }
        // we set the value at index to last and remove the last in the list.
        list.set(index, last);
        map.remove(val);
        list.remove(list.size() - 1);
        return true;
    }

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

/**
 * 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();
 */