题目链接：https://leetcode.com/problems/insert-delete-getrandom-o1/?tab=Description
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();
解题思路：初始化一个unordered_map和一个vector,unordered_map记录要插入元素在vector中的位置，删除元素时，把要删除的元素与vector最后一位元素进行交换，再删除最后一位元素。
class RandomizedSet{ public: RandomizedSet() { } bool insert(int val) { if(map.count(val)!=0) return false; map[val]=vec.size(); vec.push_back(val); return true; } bool remove(int val) { if(map.count(val)==0) return false; int pos=map[val]; map[vec.back()]=pos; swap(vec[pos],vec[vec.size()-1]); map.erase(val); vec.pop_back(); return true; } int getRandom() { return vec[rand()%vec.size()]; } private: unordered_map<int,int> map; vector<int> vec; };