Leetcode 381 Solution

class RandomizedCollection {
    map<int, list<int>> m_m;
    vector<pair<int, int*>> m_v;

public:
    /** Initialize your data structure here. */
    RandomizedCollection() {

    }

    void display()
    {
        for (auto pair : m_m)
        {
            printf("%d: ", pair.first);
            for (auto val : pair.second)
                printf("%d ", val);
            printf("\n");
        }
        for (auto pair : m_v)
            printf("%d ", pair.first);
        printf("\n\n\n");
    }

    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    bool insert(int val) {
        bool res = false;

        if (m_m.find(val) == m_m.end())
        {
            m_m[val] = list<int>();
            res = true;
        }

        m_m[val].push_back(m_v.size());
        m_v.push_back(make_pair(val, (int*)&m_m[val].back()));

        return res;
    }

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

        int index = *m_m[val].rbegin();
        swap(m_v[index], m_v[m_v.size() - 1]);

        m_v.pop_back();
        m_m[val].pop_back();
        if (m_m[val].empty())
            m_m.erase(val);

        *m_v[index].second = index;

        return true;
    }

    /** Get a random element from the collection. */
    int getRandom() {
        int index = rand() % m_v.size();
        return m_v[index].first;
    }
};

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