380. Insert Delete GetRandom O(1)--构造新类的题目

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:

// 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();
以下代码采用vector和unordered_map来实现的。
代码如下：

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(); //保存nums中的最后一个元素        m[last] = m[val]; //最后一个元素在m中的位置保存在m[val]的位置上,由于删除的是nums中的最后一个元素，所以此时要将nums中的最后一个元素last存放在原来val存放的位置上，故此句不可少        nums[m[val]] = last;  //将nums中原来val所在的位置上存储的数更新为last        nums.pop_back(); //弹出最后一个元素        m.erase(val); //删除含有val元素的项        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; //键为val，值为val所对应的位置};/** * 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(); */