380. Insert Delete GetRandom O(1)

问题描述：

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：

因为要平均时间复杂度O(1)，我们首先想到hashmap

我们可以用hashmap来存储值和下标的健值对，同时用一个数组来存储值

当我们需要插入时，可以首先通过hashmap判断是否存在，不存在再进行插入

当我们需要删除时，首先也需要判断值是否存在：

　　1. 若不存在返回false

　　2. 若存在：

　　　　a.首先将该元素与队尾元素交换（注意要更新队尾元素在hashmap中相应的下标）

　　　　b.从数组中弹出最后一个元素，从map中删除该元素。

代码：

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.count(val)) return false;
        m[val] = nums.size();
        nums.push_back(val);
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) {
        if(!m.count(val)) return false;
        int end = nums.back();
        nums[m[val]] = end;
        m[end] = m[val];
        nums.pop_back();
        m.erase(val);
        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;
};

/**
 * 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();
 */