Insert Delete GetRandom O(1)

https://leetcode.com/problems/insert-delete-getrandom-o1/description/

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();

Thoughts

由于要等概率返回任意一个数, 把插入的数放入list中, 然后直接调用random.next来生成下个随机数作为Index. 因为要O(1)时间查询和删除, 还需要一个map记录当前有什么数和每个数的位置. 当一个数要被删除, 直接把它和末尾置换.

Code

class RandomizedSet {
    Map<Integer, Integer> map;
    List<Integer> list;
    java.util.Random rand;

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

        list.add(val);
        map.put(val, list.size() - 1);
        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 loc = map.get(val);
        int lastOne = list.get(list.size() - 1);
        list.set(loc, lastOne);
        map.put(lastOne, loc);
        list.remove(list.size() - 1);
        map.remove(val);
        return true;
    }

    /** Get a random element from the set. */
    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();
 */

Analysis

每次操作时间复杂度O(1)