Implement the RandomizedSet class:

RandomizedSet() Initializes the RandomizedSet object. bool insert(int val) Inserts an item val into the set if not present. Returns true if the item was not present, false otherwise. bool remove(int val) Removes an item val from the set if present. Returns true if the item was present, false otherwise. int getRandom() Returns a random element from the current set of elements (it’s guaranteed that at least one element exists when this method is called). Each element must have the same probability of being returned. You must implement the functions of the class such that each function works in average O(1) time complexity.

Example 1:

Input [“RandomizedSet”, “insert”, “remove”, “insert”, “getRandom”, “remove”, “insert”, “getRandom”] [[], [1], [2], [2], [], [1], [2], []] Output [null, true, false, true, 2, true, false, 2]

Explanation RandomizedSet randomizedSet = new RandomizedSet(); randomizedSet.insert(1); // Inserts 1 to the set. Returns true as 1 was inserted successfully. randomizedSet.remove(2); // Returns false as 2 does not exist in the set. randomizedSet.insert(2); // Inserts 2 to the set, returns true. Set now contains [1,2]. randomizedSet.getRandom(); // getRandom() should return either 1 or 2 randomly. randomizedSet.remove(1); // Removes 1 from the set, returns true. Set now contains [2]. randomizedSet.insert(2); // 2 was already in the set, so return false. randomizedSet.getRandom(); // Since 2 is the only number in the set, getRandom() will always return 2.

<解題>

class RandomizedSet {
    List<Integer> list;
    HashMap<Integer,Integer> map;
    Random rand;

    public RandomizedSet() {
        list= new ArrayList<>();
        map = new HashMap<>();
        rand = new Random();
    }
    
    public boolean insert(int val) {
        if(map.containsKey(val)){
            return false;
        }
        map.put(val, list.size());
        list.add(val);
        return true;
    }
    
    public boolean remove(int val) {
        if(!map.containsKey(val)) return false;

        int last = list.get(list.size()-1);
        int idx = map.get(val);
        list.set(idx, last);
        map.put(last,idx);

        list.remove(list.size()-1);
        map.remove(val);
        return true;
    }
    
    public int getRandom() {
        int n = rand.nextInt(list.size());
        return list.get(n);
    }
}

T: O(1) S: O(n)

補充

public class NextInt1 { 
    public static void main(String[] args) 
    { 
  
        // create random object 
        Random ran = new Random(); 
  
        // generating integer 
        int nxt = ran.nextInt(); 
  
        // Printing the random Number 
        System.out.println 
        ("The Randomly generated integer is : " + nxt); 
    } 
} 

Random.nextInt(int n) : The nextInt(int n) is used to get a random number between 0(inclusive) and the number passed in this argument(n), exclusive.