380. Insert Delete GetRandom O(1)

map记录位置
remove时候把最后一位跟要remove的交换 然后去掉最后一位 这样可以达到O(1)

 

class RandomizedSet {
    HashMap<Integer, Integer> map;
    List<Integer> list;
    Random random = new Random();

    /** Initialize your data structure here. */
    public RandomizedSet() {
        map = 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(map.containsKey(val)){
            return false;
        }else{
            map.put(val, map.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)){
            int index = map.get(val);
            int last = list.get(list.size()-1);
            list.set(index, last);
            map.put(last, index);
            map.remove(val);
            list.remove(list.size()-1);
            return true;
        }
        return false;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        int index = random.nextInt(list.size());
        return list.get(index);
    }
}