380. Insert Delete GetRandom O(1)

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.
這道題參考了HashSet的原始碼
參考這篇部落格

class RandomizedSet {
	HashMap<Integer, Object> map;
    ArrayList<Integer> list;
	static final Object PRESENT = new Object();

	/** Initialize your data structure here. */
	public RandomizedSet() {
		map = new HashMap<Integer,
 
 Object>();
        list=new ArrayList<Integer>();
	}

	/**
	 * 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.put(val, PRESENT) == null){
            list.add(val);
            return
 
 true;
        }
		return false;
	}

	/**
	 * Removes a value from the set. Returns true if the set contained the specified
	 * element.
	 */
	public boolean remove(int val) {
        if(map.remove(val) == PRESENT){
            list.remove(Integer.valueOf(val));
            return true;
        }
		return false;
	}

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

改進方法，減少remove中的查詢時間

class RandomizedSet {

    HashMap<Integer, Integer> valToInd;
    List<Integer> list;
    int ind = 0;
    
    /** Initialize your data structure here. */
    public RandomizedSet() {
        valToInd = new HashMap<>();
        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(valToInd.containsKey(val)) return false;
        list.add(val);
        valToInd.put(val,list.size()-1);
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        int ind = valToInd.getOrDefault(val,-1);
        if(ind == -1) return false;
        Collections.swap(list,ind,list.size()-1);
        int swappedWith = list.get(ind);
        valToInd.put(swappedWith,ind);
        list.remove(list.size()-1);
        valToInd.remove(val);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        int max = list.size();
        int min = 0;
        int ind = (int)(Math.random() * (max - min) + min);
        return list.get(ind);
    }
}