Insert Delete GetRandom O(1)

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本文介绍了一种支持插入、删除及随机返回元素操作均能在平均O(1)时间内完成的数据结构实现。通过结合使用unordered_map与vector，确保了高效的元素查找、插入、删除以及随机返回功能。

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Insert Delete GetRandom O(1)

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();

解析：

unordered_map与vector结合，不能直接删除vector中某个元素，因为删除元素后的元素的下标都改变了，而map中记录的index也要跟着改变，复杂度高，用最后一个元素与当前元素交换，然后pop出最后一个元素。

代码：

class RandomizedSet {
public:
    /** Initialize your data structure here. */
    
    unordered_map<int,int>m;
    vector<int>nums;
    
    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.find(val)!=m.end()) 
        return false;
        nums.push_back(val);
        m[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 (m.find(val)==m.end())
        return false;
        int index=m[val];
        int last=nums[nums.size()-1];
        nums[index]=last;
        m[last]=index;
        m.erase(val);
        nums.pop_back();
        return true;
        
      
    }
    
    /** Get a random element from the set. */
    int getRandom() {
        return nums[rand()%nums.size()];
    }
};

/**
 * 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();
 */