Stack and Queues in Python
Last Updated : 09 May, 2022
Prerequisites :
list and
Deque in Python. Unlike C++ STL and Java Collections, Python does have specific classes/interfaces for
Stack and
Queue. Following are different ways to implement in Python
1) Using list Stack works on the principle of "Last-in, first-out". Also, the inbuilt functions in Python make the code short and simple. To add an item to the top of the list, i.e., to push an item, we use
append() function and to pop out an element we use
pop() function. These functions work quiet efficiently and fast in end operations. Let's look at an example and try to understand the working of push() and pop() function: Example:
Python3 # Python code to demonstrate Implementing # stack using list stack = ["Amar", "Akbar", "Anthony"] stack.append("Ram") stack.append("Iqbal") print(stack) # Removes the last item print(stack.pop()) print(stack) # Removes the last item print(stack.pop()) print(stack) Output: ['Amar', 'Akbar', 'Anthony', 'Ram', 'Iqbal'] Iqbal ['Amar', 'Akbar', 'Anthony', 'Ram'] Ram ['Amar', 'Akbar', 'Anthony']
Queue works on the principle of "First-in, first-out". Below is list implementation of queue. We use pop(0) to remove the first item from a list.
Python3 # Python code to demonstrate Implementing # Queue using list queue = ["Amar", "Akbar", "Anthony"] queue.append("Ram") queue.append("Iqbal") print(queue) # Removes the first item print(queue.pop(0)) print(queue) # Removes the first item print(queue.pop(0)) print(queue) Output: ['Amar', 'Akbar', 'Anthony', 'Ram', 'Iqbal'] Amar ['Akbar', 'Anthony', 'Ram', 'Iqbal'] Akbar ['Anthony', 'Ram', 'Iqbal']
2) Using Deque In case of stack, list implementation works fine and provides both append() and pop() in O(1) time. When we use deque implementation, we get same time complexity.
Python # Python code to demonstrate Implementing # Stack using deque from collections import deque queue = deque(["Ram", "Tarun", "Asif", "John"]) print(queue) queue.append("Akbar") print(queue) queue.append("Birbal") print(queue) print(queue.pop()) print(queue.pop()) print(queue) Output: deque(['Ram', 'Tarun', 'Asif', 'John']) deque(['Ram', 'Tarun', 'Asif', 'John', 'Akbar']) deque(['Ram', 'Tarun', 'Asif', 'John', 'Akbar', 'Birbal']) Birbal Akbar deque(['Ram', 'Tarun', 'Asif', 'John'])
But when it comes to queue, the above list implementation is not efficient. In queue when pop() is made from the beginning of the list which is slow. This occurs due to the properties of list, which is fast at the end operations but slow at the beginning operations, as all other elements have to be shifted one by one. So, we prefer the use of dequeue over list, which was specially designed to have fast appends and pops from both the front and back end. Let's look at an example and try to understand queue using collections.deque:
Python # Python code to demonstrate Implementing # Queue using deque from collections import deque queue = deque(["Ram", "Tarun", "Asif", "John"]) print(queue) queue.append("Akbar") print(queue) queue.append("Birbal") print(queue) print(queue.popleft()) print(queue.popleft()) print(queue) Output: deque(['Ram', 'Tarun', 'Asif', 'John']) deque(['Ram', 'Tarun', 'Asif', 'John', 'Akbar']) deque(['Ram', 'Tarun', 'Asif', 'John', 'Akbar', 'Birbal']) Ram Tarun deque(['Asif', 'John', 'Akbar', 'Birbal'])
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