Design a data structure that supports all following operations in average O(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.

設計一種數據結構，支持在O(1)時間的下述操作：

1. insert(val): 在不含該元素的情況下插入元素
2. remove(val): 在含有元素的情況下移除元素
3. getRandom: 從當前元素集中返回隨機元素，每個元素被取得的概率必須均等

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();

思路：
題目要求O(1)時間內進行操作，這里采用vector存儲元素，用unordered_map<int,int>存儲元素和它們的地址。

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

    }

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

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) {
        if(m.find(val)==m.end()) return false;
        int last=nums.back();
        m[last]=m[val];
        nums[m[val]]=last;
        nums.pop_back();
        m.erase(val);
        return true;
    }

    /** Get a random element from the set. */
    int getRandom() {
        return nums[rand()% nums.size()];
    }
private:
    vector<int> nums;
    unordered_map<int,int> m;
};

/**
 * 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();
 */