Insert Delete GetRandom O(1)

Desicription

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:

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// 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();

Solution

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class RandomizedSet {
private:
    std::unordered_map<int, int> map = std::unordered_map<int, int>();
    std::vector<int> vector = std::vector<int>();
public:
    /** Initialize your data structure here. */
    RandomizedSet() = default;

    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    bool insert(int val) {
        if(map.find(val) != map.end()) {
            return false;
        }
        vector.push_back(val);
        map[val] = vector.size() - 1;
        return true;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) {
        if(map.find(val) == map.end()) {
            return false;
        }

        int last = vector.back();
        map[last] = map[val];
        vector[map[last]] = last;
        map.erase(val);
        vector.pop_back();
        return true;
    }

    /** Get a random element from the set. */
    int getRandom() {
        return vector[rand() % vector.size()];
    }
};

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet* obj = new RandomizedSet();
 * bool param_1 = obj->insert(val);
 * bool param_2 = obj->remove(val);
 * int param_3 = obj->getRandom();
 */