381. Insert Delete GetRandom O(1) - Duplicates allowed
Design a data structure that supports all following operations in average O(1) time.
Note: Duplicate elements are allowed.
insert(val)
: Inserts an item val to the collection.remove(val)
: Removes an item val from the collection if present.getRandom
: Returns a random element from current collection of elements. The probability of each element being returned is linearly related to the number of same value the collection contains.
Example:
// Init an empty collection. RandomizedCollection collection = new RandomizedCollection(); // Inserts 1 to the collection. Returns true as the collection did not contain 1. collection.insert(1); // Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1]. collection.insert(1); // Inserts 2 to the collection, returns true. Collection now contains [1,1,2]. collection.insert(2); // getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3. collection.getRandom(); // Removes 1 from the collection, returns true. Collection now contains [1,2]. collection.remove(1); // getRandom should return 1 and 2 both equally likely. collection.getRandom();
错误代码:主要原因是相同值的删除时会导致原先的顺序发生改变,导致每次碰到重复的时候都会只操作最后一个。
错误用例:
["RandomizedCollection","insert","insert","insert","insert","insert","insert","remove","remove","remove","remove","getRandom","getRandom","getRandom","getRandom","getRandom","getRandom","getRandom","getRandom","getRandom","getRandom"]
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class RandomizedCollection { public: /** Initialize your data structure here. */ RandomizedCollection() { } /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */ bool insert(int val) { nums.emplace_back(val); hashTable[val].emplace_back(nums.size()-1); return hashTable[val].size()==1; } /** Removes a value from the collection. Returns true if the collection contained the specified element. */ bool remove(int val) { if(hashTable.find(val)==hashTable.end())return false; int last=nums.back(); if(val!=last){ // Reduce redundant access. nums[hashTable[val].back()]=last; hashTable[last].back()=hashTable[val].back(); } hashTable[val].pop_back(); nums.pop_back(); if(hashTable[val].size()==0)hashTable.erase(val); return true; } /** Get a random element from the collection. */ int getRandom() { return nums[rand()%nums.size()]; } private: unordered_map<int,vector<int> >hashTable; vector<int>nums; }; /** * Your RandomizedCollection object will be instantiated and called as such: * RandomizedCollection obj = new RandomizedCollection(); * bool param_1 = obj.insert(val); * bool param_2 = obj.remove(val); * int param_3 = obj.getRandom(); */
正确解法:
class RandomizedCollection { public: /** Initialize your data structure here. */ RandomizedCollection() { } /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */ bool insert(int val) { hashTable[val].emplace_back(nums.size()); nums.emplace_back(val,hashTable[val].size()-1); return hashTable[val].size()==1; } /** Removes a value from the collection. Returns true if the collection contained the specified element. */ bool remove(int val) { if(hashTable.find(val)==hashTable.end())return false; auto last=nums.back(); hashTable[last.first][last.second]=hashTable[val].back(); nums[hashTable[val].back()]=last; hashTable[val].pop_back(); nums.pop_back(); if(hashTable[val].size()==0)hashTable.erase(val); return true; } /** Get a random element from the collection. */ int getRandom() { return nums[rand()%nums.size()].first; } private: unordered_map<int, vector<int> >hashTable; vector<pair<int, int> >nums; }; /** * Your RandomizedCollection object will be instantiated and called as such: * RandomizedCollection obj = new RandomizedCollection(); * bool param_1 = obj.insert(val); * bool param_2 = obj.remove(val); * int param_3 = obj.getRandom(); */