380. Insert Delete GetRandom O(1)

问题：

设计数据结构，使得以下三个方法的时间复杂度都为O(1)

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();

　　

解法：

本题需让insert，remove的复杂度为O(1)

数组的insert方法复杂度满足，remove中的查找不满足，因此加入hash来帮助记录消化查找的代价。

vector<int> nums

unordered_map<int, int>  //(key: nums[i], value: i)

代码参考：

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(mapnums.find(val)!=mapnums.end()){
            return false;
        }
        nums.push_back(val);
        mapnums[val]=nums.size()-1;
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) {
        if(mapnums.find(val)==mapnums.end()){
            return false;
        }
        int last=nums.back();
        nums[mapnums[val]]=last;
        mapnums[last]=mapnums[val];
        nums.pop_back();
        mapnums.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> mapnums;
};

/**
 * 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();
 */