380_insert_delete_getrandom_O(1).java

class RandomizedSet {

    private List<Integer> list;
    private Map<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) == true) {
            return false;
        } else {
            list.add(val);
            map.put(val, list.size()-1);
            return true;
        }
    }
    
    public boolean remove(int val) {
        if (map.containsKey(val) == false) {
            return false;
        } else {
            int index = map.get(val);
            int lastVal = list.get(list.size()-1);
            list.set(index, lastVal);
            map.put(lastVal, index);
            list.remove(list.size()-1);
            map.remove(val);
            return true;
        }
    }
    
    public int getRandom() {
        return list.get(rand.nextInt(list.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();
 */

/**
 * This is my personal record of solving Leetcode Problems. 
 * If you have any questions, please discuss them in [Issues](https://github.com/mengxinayan/leetcode/issues).
 * Copyright (C) 2020-2022  mengxinayan
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */