380. Insert Delete GetRandom O(1)
问题:
设计数据结构,使得以下三个方法的时间复杂度都为O(1)
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();
解法:
本题需让insert,remove的复杂度为O(1)
数组的insert方法复杂度满足,remove中的查找不满足,因此加入hash来帮助记录消化查找的代价。
vector<int> nums
unordered_map<int, int> //(key: nums[i], value: i)
代码参考:
1 class RandomizedSet { 2 public: 3 /** Initialize your data structure here. */ 4 RandomizedSet() { 5 6 } 7 8 /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */ 9 bool insert(int val) { 10 if(mapnums.find(val)!=mapnums.end()){ 11 return false; 12 } 13 nums.push_back(val); 14 mapnums[val]=nums.size()-1; 15 return true; 16 } 17 18 /** Removes a value from the set. Returns true if the set contained the specified element. */ 19 bool remove(int val) { 20 if(mapnums.find(val)==mapnums.end()){ 21 return false; 22 } 23 int last=nums.back(); 24 nums[mapnums[val]]=last; 25 mapnums[last]=mapnums[val]; 26 nums.pop_back(); 27 mapnums.erase(val); 28 return true; 29 } 30 31 /** Get a random element from the set. */ 32 int getRandom() { 33 return nums[rand() % nums.size()]; 34 } 35 36 private: 37 vector<int> nums; 38 unordered_map<int, int> mapnums; 39 }; 40 41 /** 42 * Your RandomizedSet object will be instantiated and called as such: 43 * RandomizedSet* obj = new RandomizedSet(); 44 * bool param_1 = obj->insert(val); 45 * bool param_2 = obj->remove(val); 46 * int param_3 = obj->getRandom(); 47 */