How to remove set of lists from a list of lists

Question

I have 2 lists of strings

List1 = [["Hey there"], ["hi"], ["hello"]]
List2 = [["hi"], ["hello"]]

Is there a O(n) way to remove elements of List2 from List1 ?

Desired output = [["Hey there"]]

Answer 1

You can do this with two O(n) steps:

List2_set = set(map(tuple, List2))
List1_filtered = [row for row in List1 if tuple(row) not in List2_set]

1. Convert the list of items to exclude to a set of tuples

   • This conversion is O(n)
   • set is required because checking membership for sets is O(1) instead of O(n)
   • tuple is required for set items instead of list , because tuple is hashable

2. Check membership for each element of List1

   • This check is also O(n)
   • The set collection uses a hash table to allow O(1) membership testing

The total then is O(n) + O(n) => O(n), that is, the performance is linear with number of total elements of List1 + List2 .