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Leetcode 380 Insert Delete Getrandom O1

发表于 更新于 分类于 Valine：
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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 (it’s guaranteed that at least one element exists when this method is called). Each element must have the same probability of being returned.

Example:

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// 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();

vector + hashmap

vector里面储存各个元素, hashmap储存每一个元素对应的数组下标

  • insert时,先在vector后面pushback某个value, 再从hashmap中添加一个这个值到数组下标位置的映射(value => vector.size() - 1)
  • remove时, 先从hashmap中查找val对应的映射(val => valIndex), 和数组最后一个元素对应的映射, (back => backIndex) , 知道val在数组中的位置valIndex后交换val和数组最后一个元素, swap(vc[index], vc.back());同时在hashmap上登记相应的变动 swap(itBack->second, itVal->second); 然后删除数组中最后一个元素, 删除hashmap中对应最后一个元素的映射
  • getRandom, 选择一个[0, vc.size)的随机数作为随即下标, 返回对应的值即可
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class RandomizedSet {
private:
    vector<int> vc;
    unordered_map<int,int> mp;
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) {
        bool hasVal = (mp.find(val) != mp.end());
        if(hasVal)
            return false;
        vc.push_back(val);
        mp.insert({val,vc.size()-1});
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) {
        unordered_map<int, int>::iterator itVal = mp.find(val);
        if(itVal == mp.end())
            return false;
        unordered_map<int, int>::iterator itBack = mp.find(vc.back());
        int index = itVal->second;
        swap(vc[index], vc.back());
        swap(itBack->second, itVal->second);
        vc.pop_back();
        mp.erase(itVal);
        return true;
    }
    
    /** Get a random element from the set. */
    int getRandom() {
        int randomIndex = rand() % vc.size();
        return vc[randomIndex];
    }
};

/**
 * 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();
 */