What is the difference between a stack and a queue?

1 What is a stack?

A stack is a data structure that follows the principle of last-in, first-out (LIFO). This means that the last item that is added to the stack is the first one that is removed from it. You can think of a stack as a pile of books or plates, where you can only access the topmost element. To add an item to the stack, you use the push operation. To remove an item from the stack, you use the pop operation. Stacks are useful for storing data that needs to be accessed in reverse order, such as undoing actions, navigating web pages, or evaluating expressions.

2 What is a queue?

A queue is a data structure that follows the principle of first-in, first-out (FIFO). This means that the first item that is added to the queue is the first one that is removed from it. You can think of a queue as a line of people or cars, where you can only access the frontmost element. To add an item to the queue, you use the enqueue operation. To remove an item from the queue, you use the dequeue operation. Queues are useful for storing data that needs to be accessed in the same order as it was entered, such as processing requests, scheduling tasks, or buffering data.

3 Advantages and disadvantages

Stacks and queues each have their own benefits and drawbacks, depending on the type and complexity of the data and the operations you need to perform on it. Stacks are simple and fast to implement, but have limited capacity and can cause memory overflow if not managed properly. On the other hand, queues are flexible and efficient for handling sequential data, but require more space and time to maintain and access, as well as can cause underflow if not checked regularly. Stacks are great for recursive algorithms, backtracking, and depth-first search, while queues are better for iterative algorithms, level-order traversal, and breadth-first search. However, stacks are not suitable for breadth-first search, sorting, or parallel processing; likewise, queues are not suitable for depth-first search, reversing, or stack-based languages.

4 Examples in data science

Stacks and queues are widely used in data science, particularly for data structure and algorithm problems. For instance, stacks can be used to evaluate postfix or prefix expressions, which are common in mathematical and logical operations. For example, to evaluate the postfix expression 2 3 + 4 *, you can use a stack to push and pop the operands and operators, and get the result 20. Queues can be employed for breadth-first search, which is a technique for exploring graphs or trees. To find the shortest path from node A to node D in a graph, you can use a queue to store and visit the adjacent nodes, and keep track of the distance and the parent of each node. Additionally, stacks can be used to implement depth-first search. To find all possible paths from node A to node D in a graph, you can use a stack to store and visit the next nodes, and backtrack when you reach a dead end or the destination. Lastly, queues can be used to implement priority queues, which are data structures that store data according to their priority or importance. To schedule tasks in a CPU, you can use a priority queue to assign each task a priority based on its deadline, and process the tasks in the order of their priority.

5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?