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();

Link:

https://leetcode.com/problems/insert-delete-getrandom-o1/#/description

解题方法：

哈希表可以O(1)时间增加删除，vector可以在O(1)时间取得随机数、删除尾部元素、在尾部增加元素。
如果用哈希表记录vector里面元素的位置，在删除的时候将vector需要删除的元素与其尾部互换，就可以在O(1)时间删除该元素。

Time Complexity：

O(1)

完整代码：

class RandomizedSet 
{
private:
    vector<int> cache;
    unordered_map <int, int> hash;
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(hash.find(val) != hash.end())
            return false;
        cache.push_back(val);
        hash[val] = cache.size() - 1;
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) 
    {
        if(hash.find(val) == hash.end())
            return false;
        hash[cache.back()] = hash[val]; //交换位置之前哈希表中对应的位置也要交换
        swap(cache[cache.size() - 1], cache[hash[val]]);
        cache.pop_back();
        hash.erase(val);
        return true;
    }
    
    /** Get a random element from the set. */
    int getRandom() 
    {
        return cache[rand()%cache.size()];
    }
};