You are tasked with designing an application that includes both graphical rendering of cursors in the user interface, as well as text-based interfaces to control them. The current implementation does not use any programming language or library outside of a simple text editor, so you need to choose your preferred tool and explain about how this could be accomplished using Python 3.5. Your answer: You are designing an application for Microsoft Word that can parse all the files in a folder named "pictures". You want to find out whether it is possible to use regex to search through each file in each subdirectory of the folder and retrieve only those which have .txt extension. The current time complexity should be O(n) or less than O(n). For all these operations, you can use any programming language.