两张哈希表

• 有重复的不重复加

• 因为有remove行为，那么map里的index就可能会不连续，出现很多洞，那么我getRandom的时候就麻烦了，所以比如用户想remove掉15

我拿24去填15的洞， 并且继承15的index，(我拿最后一条记录去填洞，可以保证每次index都连续)

 class RandomizedSet {
        private HashMap<Integer, Integer> keyIndexMap;
        private HashMap<Integer, Integer> indexKeyMap;
        private int size;
        public RandomizedSet() {
            keyIndexMap = new HashMap<>();
            indexKeyMap = new HashMap<>();
            size = 0;
        }
        public boolean insert(int val) {
            if (!keyIndexMap.containsKey(val)) {
                keyIndexMap.put(val, size);
                indexKeyMap.put(size++, val);
                return true;
            }
            return false;
        }
        public boolean remove(int val) {
            if (keyIndexMap.containsKey(val)) {
                int deleteIndex = keyIndexMap.get(val);
                int lastIndex = --size;
                int lastKey = indexKeyMap.get(lastIndex);
                keyIndexMap.put(lastKey, deleteIndex);
                indexKeyMap.put(deleteIndex, lastKey);
                keyIndexMap.remove(val);
                indexKeyMap.remove(lastIndex);
                return true;
            }
            return false;
        }
        public int getRandom() {
            if (size == 0) {
                return -1;
            }
            int randomIndex = (int) (Math.random() * size);
            return indexKeyMap.get(randomIndex);
        }
    }