问题
IDictionary<TK, TV> defines method IDictionary.ContainsKey(in TK) and property IDictionary.Keys (of type ICollection).
I'm interested in (asymptotic) complexity of this method and property in Dictionary implementation of IDictionary<TK, TV>.
Consider definitions
IDictionary<int, string> dict = new Dictionary<int, string>();
int k = new Random().Next();
and consider that we have added to the dictionary dict n unique KeyValuePairs.
What is expected (asymptotic) complexity of a call dict.ContainsKey(k)? I hope it's in O(1) but I did not find it in documentation of Dictionary.ContainsKey.
What is expected (asymptotic) complexity of call dict.Keys.Contains(k)? I hope it's in O(1)but I did not find it in documentation of Dictionary.Keys nor in documentation of Dictionary.KeyCollection. If it is in O(1) I don't understand why type of IDictionary.Keys property is ICollection and not ISet (like in Java for example).
回答1:
Since IDictionary<K,V> is only an interface, not an implementation, then it doesn't provide any performance guarantees.
For the built-in Dictionary<K,V> type, the ContainsKey method should be O(1):
This method approaches an O(1) operation.
The Keys.Contains method actually calls the parent dictionary's ContainsKey method, so that should also be O(1):
This method is an O(1) operation.
(Both quotes are taken from the "Remarks" section of the relevant documentation pages.)
回答2:
The first link you provided says, in the Remarks:
This method approaches an O(1) operation.
Also, if you click on the Contains method you'll see the same thing in the remarks:
This method is an O(1) operation.
来源：https://stackoverflow.com/questions/5671512/how-fast-is-operation-on-keycollection-returned-by-dictionary-keys-net