https://leetcode.com/problems/insert-delete-getrandom-o1/
please comment on performance
Implement the RandomizedSet class:
bool insert(int val) Inserts an item val into the set if not present.Returns true if the item was not present, false otherwise. boolremove(int val) Removes an item val from the set if present. Returnstrue if the item was present, false otherwise. int getRandom() Returnsa random element from the current set of elements (it's guaranteedthat at least one element exists when this method is called). Eachelement must have the same probability of being returned. Follow up:Could you implement the functions of the class with each functionworks in average O(1) time?
Example 1:
Input ["RandomizedSet", "insert", "remove", "insert", "getRandom","remove", "insert", "getRandom"] [[], [1], [2], [2], [], [1], [2], []]Output [null, true, false, true, 2, true, false, 2]
Explanation RandomizedSet randomizedSet = new RandomizedSet();randomizedSet.insert(1); // Inserts 1 to the set. Returns true as 1was inserted successfully. randomizedSet.remove(2); // Returns falseas 2 does not exist in the set. randomizedSet.insert(2); // Inserts 2to the set, returns true. Set now contains [1,2].randomizedSet.getRandom(); // getRandom() should return either 1 or 2randomly. randomizedSet.remove(1); // Removes 1 from the set, returnstrue. Set now contains [2]. randomizedSet.insert(2); // 2 was alreadyin the set, so return false. randomizedSet.getRandom(); // Since 2 isthe only number in the set, getRandom() will always return 2.
Constraints:
\$-2^{31} <= val <= 2^{31} - 1\$. At most\$10^5\$ calls will be made to insert, remove, and getRandom. There will be at least one element in the datastructure when getRandom is called.
public class RandomizedSet { private HashSet<int> _set; /** Initialize your data structure here. */ public RandomizedSet() { _set = new HashSet<int>(); } /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */ public bool Insert(int val) { if (_set.Contains(val)) { return false; } _set.Add(val); return true; } /** Removes a value from the set. Returns true if the set contained the specified element. */ public bool Remove(int val) { if (_set.Contains(val)) { _set.Remove(val); return true; } return false; } /** Get a random element from the set. */ public int GetRandom() { Random rand = new Random(); int key = rand.Next(_set.Count); return _set.ElementAt(key); }}/** * 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(); */
2 Answers2
- The
HashSet<int>won't be changed hence make itreadonly. - Instead of calling
Contains()prior to the call toAdd(), if this evaluates tofalse, can be simplified to justreturn _set.Add(val);because theAdd()method returnsfalseif the value is allready in theHashSet.Reference - Instead of calling
Contains()prior to callingRemove()can be simplified as well to justreturn _set.Remove(val);becauseRemove()will returnfalseif the item isn't in theHashSet.Reference - Calling
GetRandom()in repeatedly short order may result in the same element because theSeedof a createdRandomin .NET framework is based on the current timestamp. It is better to create a class-levelRandomto be used.
Summing up leads to
public class RandomizedSet { private readonly HashSet<int> _set; /** Initialize your data structure here. */ public RandomizedSet() { _set = new HashSet<int>(); } /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */ public bool Insert(int val) { return _set.Add(val); } /** Removes a value from the set. Returns true if the set contained the specified element. */ public bool Remove(int val) { return _set.Remove(val); } private readonly Random rand = new Random(); /** Get a random element from the set. */ public int GetRandom() { int key = rand.Next(_set.Count); return _set.ElementAt(key); }}
GetRandom() Complexity
HashSet<T> does not support lookup by index soElementAt needs to iterate until the requested element is reached. That requires O(n) steps not O(1).
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