[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;
};