380.insert-delete-get-random-o-1.java

/*
 * @lc app=leetcode id=380 lang=java
 *
 * [380] Insert Delete GetRandom O(1)
 *
 * https://leetcode.com/problems/insert-delete-getrandom-o1/description/
 *
 * algorithms
 * Medium (41.99%)
 * Likes:    1152
 * Dislikes: 90
 * Total Accepted:    117.7K
 * Total Submissions: 273.9K
 * Testcase Example:  '["RandomizedSet","insert","remove","insert","getRandom","remove","insert","getRandom"]\n[[],[1],[2],[2],[],[1],[2],[]]'
 *
 * 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.
 *
 *
 *
 * Example:
 *
 * // Init an empty set.
 * RandomizedSet randomSet = new RandomizedSet();
 *
 * // Inserts 1 to the set. Returns true as 1 was inserted successfully.
 * randomSet.insert(1);
 *
 * // Returns false as 2 does not exist in the set.
 * randomSet.remove(2);
 *
 * // Inserts 2 to the set, returns true. Set now contains [1,2].
 * randomSet.insert(2);
 *
 * // getRandom should return either 1 or 2 randomly.
 * randomSet.getRandom();
 *
 * // Removes 1 from the set, returns true. Set now contains [2].
 * randomSet.remove(1);
 *
 * // 2 was already in the set, so return false.
 * randomSet.insert(2);
 *
 * // Since 2 is the only number in the set, getRandom always return 2.
 * randomSet.getRandom();
 *
 *
 */
class RandomizedSet {

  List<Integer> list;
  Map<Integer, Integer> map;
  Random random;

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

  /** 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;
    }
    map.put(val, list.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)) {
      return false;
    }
    int index = map.get(val);
    if (index < list.size() - 1) {
      int last = list.get(list.size() - 1);
      map.put(last, index);
      list.set(index, last);
    }
    map.remove(val);
    list.remove(list.size() - 1);
    return true;
  }

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

/**
 * 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();
 */