Insert Delete GetRandom O(1) - Duplicates allowed

题目来源
和上一题有点类似，求插入删除取随机O(1)完成，区别在于可以重复。所以maps中需要vector来存储，然后nums中需要存一下这个元素在maps中的vector中的哪个位置。题目不难，就是比较绕。

class RandomizedCollection {
public:
    /** Initialize your data structure here. */
    RandomizedCollection() {
        
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    bool insert(int val) {
        bool res = true;
        if (maps.count(val) != 0)
            res = false;
        nums.push_back(make_pair(val, maps[val].size()));
        maps[val].push_back(nums.size()-1);
        return res;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    bool remove(int val) {
        if (maps.count(val) != 0) {
            maps[nums.back().first][nums.back().second] = maps[val].back();
            nums[maps[val].back()] = nums.back();
            nums.pop_back();
            maps[val].pop_back();
            if (maps[val].size() == 0)
                maps.erase(val);
            return true;
        }
        else
            return false;
    }
    
    /** Get a random element from the collection. */
    int getRandom() {
        return nums[rand() % nums.size()].first;
    }
private:
    unordered_map> maps;
    vector> nums;
};

/**
 * 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();
 */