380. Insert Delete GetRandom O(1)

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();

刚开始想的是用set，但用到getRandom()时，会用到循环，时间不是O(n)

class RandomizedSet {    set<int> nums;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(nums.find(val)!=nums.end()) return 0;       nums.insert(val);       return 1;    }        /** Removes a value from the set. Returns true if the set contained the specified element. */    bool remove(int val) {      if(nums.find(val)==nums.end())return 0;       nums.erase(val);       return 1;    }        /** Get a random element from the set. */    int getRandom() {       int rnd=rand()%nums.size(),i=0;       set<int>::iterator it;       for( it=nums.begin();i!=rnd;it++,i++);//这里用循环了，会慢       return *it;    }};

转载别人的：点击打开链接

用一个hash表记录每个元素的值在vector中的位置，当要删除一个元素时，先找到该元素的位置，将其更新为末尾元素，vector末尾弹出，同时更新hash表。

class RandomizedSet {    //set<int> nums;    vector<int> nums;    map<int ,int> locs;//vals to keypublic:    /** 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(nums.find(val)!=nums.end()) return 0;       nums.insert(val);       return 1;*/       if(locs.count(val)!=0) return 0;       nums.push_back(val);       locs[val]=(nums.size()-1);       return 1;    }        /** Removes a value from the set. Returns true if the set contained the specified element. */    bool remove(int val) {     /* if(nums.find(val)==nums.end())return 0;       nums.erase(val);       return 1;*/       if(locs.count(val)==0) return 0;             int last=nums[nums.size()-1];       int loc=locs[val];       nums[loc]=last;//更新nums       locs[last]=loc;//更新locs       locs.erase(val);       nums.pop_back();       return 1;    }        /** Get a random element from the set. */    int getRandom() {      /* int rnd=rand()%nums.size(),i=0;       set<int>::iterator it;       for( it=nums.begin();i!=rnd;it++,i++);       return *it;*/              return nums[rand()%nums.size()];    }};