I'm looking for a data structure that will allow me to test if an element exists within it in O(1) time as such: if value in my_data_structure: pass However, I need it to be able to keep its order of inserted elements, so I can access them via index or any other way. The closest I found from a similar question was a multiset, to which in Python a Counter is the closest, but those don't keep any semblance of order and the duplicates are 'aggregated'. Does anything like this exist at all, even outside of Python, so I could program it myself? Continue reading...
