Calculate time complexity for singly_linked_list.py

[wangster6]

Methods that need calculations for:

• is_empty
• append
• prepend
• len

Comment time complexity in the same format as the one I calculated for the delete method. If there are multiple scenarios in the method, calculate for each method and comment them all.

[wangster6]

Assigning this to you @tea-na, take your time and do it whenever you have free time. Starting it off easy since its just a singly linked list so I don't think it should take any longer than 5-10 minutes. Thanks!