[leetcode]380. Insert Delete GetRandom O(1)j

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.

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.

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;};