Stacks

A stack data structure is a so-called LIFO (Last In, First Out) data structure. It means that the last item inserted in the stack will be the first item removed. And, of course, it is very important that we can access only the last element in a data structure exclusively when dealing with the stack.

Stack implementation


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import numpy as np

class Stack:
    def __init__(self):
        self.stack = np.array([])  # Initialize an empty NumPy array

    def push(self, item):
        self.stack = np.append(self.stack, item)  # Append the item to the end of the array

    def pop(self):
        if not self.is_empty():
            return_value = self.stack[-1]  # Store the value to return
            self.stack = np.delete(self.stack, -1)  # Remove the last item from the array
            return return_value  # Return the removed item
        else:
            return None  # Return None if the stack is empty


# Example usage:
stack = Stack()

stack.push(1)
stack.push(2)
stack.push(3)

print("Stack:", stack.stack)

print("Pop:", stack.pop())
print("Stack after pop:", stack.stack)

It is important to remember that a stack is an abstract data type. It means that it only defines behavior but not a concrete implementation. The stack abstract data type can be implemented either with the help of the array or linked list data structure.

Usually, in concrete implementations of the stack, two basic methods are used:

push() - inserts an item into a stack;
pop() - removes the last inserted item from a stack.

Basic operations time complexity

Everything was clear?

Section 3. Chapter 1