657 · Insert Delete GetRandom O(1)
Algorithms
Medium

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();
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Related Problems

954
Insert Delete GetRandom O(1) - Duplicates allowed
Hard

解法1：vector + unordered_map

class RandomizedSet {
    
    
public:
    RandomizedSet() {
    
    
        srand(time(NULL));
    }

    /*
     * @param val: a value to the set
     * @return: true if the set did not already contain the specified element or false
     */
    bool insert(int val) {
    
    
        if (locate.find(val) == locate.end()) {
    
    
            data.push_back(val);
            locate[val] = data.size() - 1;
        } 
    }

    /*
     * @param val: a value from the set
     * @return: true if the set contained the specified element or false
     */
    bool remove(int val) {
    
    
        if (locate.find(val) == locate.end()) return false;
        int pos = locate[val];
        locate.erase(val);
        data[pos] = data[data.size() - 1];
        locate[data[pos]] = pos;
        data.pop_back();
        return true;
    }

    /*
     * @return: Get a random element from the set
     */
    int getRandom() {
    
    
        int rand_num = rand() % data.size();
        return data[rand_num];
    }
private:
    vector<int> data;
    unordered_map<int, int> locate;
};

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * bool param = obj.insert(val);
 * bool param = obj.remove(val);
 * int param = obj.getRandom();
 */