380 Insert Delete GetRandom O(1)

380. Insert Delete GetRandom O(1)

1. Question

Design a data structure that supports all following operations inaverageO(1)time.

1. insert(val): Inserts an item val to the set if not already present.

2. remove(val): Removes an item val from the set if present.

3. 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();

2. Implementation

(1) Hash Table

class RandomizedSet {
    List<Integer> nums;
    Map<Integer, Integer> map;
    Random rand;

    /** Initialize your data structure here. */
    public RandomizedSet() {
        nums = new ArrayList<>();
        map = new HashMap<>();
        rand = 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, nums.size());
        nums.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 < nums.size() - 1) {
            int lastNum = nums.get(nums.size() - 1);
            map.put(lastNum, index);
            nums.set(index, lastNum);
        }
        map.remove(val);
        nums.remove(nums.size() - 1);
        return true;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        return nums.get(rand.nextInt(nums.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();
 */

3. Time & Space Complexity

时间复杂度: insert: O(1), remove: O(1), getRandom: O(1)

空间复杂度: O(n), n是插入的数的个数