lintcode: Insert Delete GetRandom O(1)

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.

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class RandomizedSet {    private:    vector<int> nums;    unordered_map<int, int> m;    public:    RandomizedSet() {        // do initialize if necessary    }        // Inserts a value to the set    // Returns true if the set did not already contain the specified element or false    bool insert(int val) {        // Write your code here        if (m.count(val))            return false;                    nums.push_back(val);        m[val] = nums.size()-1;        return true;    }        // Removes a value from the set    // Return true if the set contained the specified element or false    bool remove(int val) {        // Write your code here                if (!m.count(val))            return false;                m[nums[nums.size()-1]] = m[val];        nums[m[val]] = nums[nums.size()-1];                nums.pop_back();        m.erase(val);        return true;    }        // Get a random element from the set    int getRandom() {        // Write your code here                return nums[rand() % nums.size()];    }};/** * Your RandomizedSet object will be instantiated and called as such: * RandomizedSet obj = new RandomizedSet(); * bool param = obj.insert(val); * bool param = obj.remove(val); * int param = obj.getRandom(); */