380. Insert Delete GetRandom O(1) && 381. Insert Delete GetRandom O(1) - Duplicates allowed

380. Insert Delete GetRandom O(1)

Design a data structure that supports all following operations in average O(1) time. 

1. insert(val): Inserts an item val to the set if not already present.
2. remove(val): Removes an item val from the set if present.
3. 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();

 Array Hash Table Design

 

public class RandomizedSet {
  private ArrayList<Integer> nums = new ArrayList<>();
  private HashMap<Integer, Integer> locations = new HashMap<>();
  private java.util.Random rand = new java.util.Random();

  /**
   * 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 = locations.containsKey(val);
    if (contain)
      return false;
    locations.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) {
    Integer index = locations.get(val);
    if (index == null)
      return false;
    //swap this val to the end of the array before removing.
    //otherwise, it is not an O(1) operation for deleting the element.
    if (index < nums.size() - 1) {
      int lastVal = nums.get(nums.size() - 1);
      nums.set(index, lastVal);
      locations.put(lastVal, index);
    }
    locations.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()));
  }
}

/**
 * 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();
 */

 

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. 

1. insert(val): Inserts an item val to the collection.
2. remove(val): Removes an item val from the collection if present.
3. 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.
RandomizedCollection collection = new RandomizedCollection();
// 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();

 Array Hash Table Design

 

 

The only change from problem 380 is that we use a HashSet to keep different locations for the same value, and update the hashmap when inserting and removing.

 

public class RandomizedCollection {
  private ArrayList<Integer> nums = new ArrayList<>();
  //use a deque to keep all locaionMap of the same value, and remove them one by one
  private HashMap<Integer, Set<Integer>> locaionMap = new HashMap<>();
  private java.util.Random rand = new java.util.Random();

  /**
   * Inserts a value to the set. Returns true if the set did not already contain the specified element.
   */
  public boolean insert(int val) {
    Set<Integer> locs = locaionMap.get(val);
    boolean contains = locs != null;
    if (!contains) {
      locs = new HashSet<>();
      locaionMap.put(val, locs);
    }
    locs.add(nums.size());
    nums.add(val);
    return contains;
  }

  /**
   * Removes a value from the set. Returns true if the set contained the specified element.
   */
  public boolean remove(int val) {
    Set<Integer> valLocations = locaionMap.get(val);
    if (valLocations == null)
      return false;
    
    int index = valLocations.iterator().next();  
    //must remove before the if-statement, e.g have inserted (3,3)
    //we will swap 3 and 3, and remove the last index(1) from lastValLocations, and then add 0
    //but 0 is already there if we don't remove it here.
    valLocations.remove(index); //You will see the bug if you move this statement after the if block
    
    //swap this val to the end of the array before removing.
    //otherwise, it is not an O(1) operation for deleting the element.
    if (index < nums.size() - 1) {
      int lastVal = nums.get(nums.size() - 1);
      Set<Integer> lastValLocations = locaionMap.get(lastVal);
      nums.set(index, lastVal); //move lastVal to index position
      //update the location map for lastVal
      lastValLocations.remove(nums.size() - 1);
      lastValLocations.add(index);
    }
    
    if (valLocations.isEmpty())
      locaionMap.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()));
  }
}


/**
 * Your RandomizedCollection object will be instantiated and called as such:
 * RandomizedCollection obj = new RandomizedCollection();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */