Initial State in Bracket Balancing: Empty Stack

Consider the following sequence of operations on a data structure that behaves in a 'last-in, first-out' manner. What will be the final contents of the data_structure list after all the operations are executed?

data_structure = []
data_structure.append('A')
data_structure.append('B')
data_structure.pop()
data_structure.append('C')
data_structure.append('D')
data_structure.pop()

Implementing an 'Undo' Feature

Consider the following Python code snippet which uses a list to simulate a data structure that operates on a 'last-in, first-out' (LIFO) principle. Arrange the following states of the items list in the chronological order they occur as the code executes from top to bottom.

items = []
items.append(42)      # State 1
items.append(15)      # State 2
items.pop()           # State 3
items.append(88)      # State 4