A Common-Sense Guide to Data Structures and Algorithms in Python, Volume 1

Stacks

A stack stores data in the same way arrays do—it’s simply a list of elements. The one catch is that stacks have the following three constraints:

• Data can be inserted only at the end of a stack.
• Data can be deleted only from the end of a stack.
• Only the last element of a stack can be read.

You can think of a stack as an actual stack of dishes; you can’t look at the face of any dish other than the one at the top. Similarly, you can’t add any dish except to the top of the stack, nor can you remove any dish besides the one at the top. (At least, you shouldn’t.) In fact, most computer science literature refers to the end of the stack as its top and the beginning of the stack as its bottom.

Our diagrams will reflect this terminology by viewing ...