A stack is a fundamental data structure in computer science that follows the Last-In-First-Out (LIFO) principle. It is like a physical stack of plates where the last plate placed on top is the first one to be removed.
In programming, a stack allows operations only at one end – the top. This end is known as the “top of the stack”.
Basic Operations on a Stack
A stack supports three main operations:
- Push: This operation adds an element to the top of the stack.
- Pop: This operation removes and returns the topmost element from the stack.
- Peek: This operation returns the topmost element from the stack without removing it.
These operations make stacks useful for solving problems that involve reversing or tracking elements in a specific order.
Implementation of Stacks
Stacks can be implemented using arrays or linked lists. In both cases, we need to keep track of the top element using a pointer or an index.
Array Implementation
In array implementation, we create an array with a fixed size and maintain an index variable that points to the top element. When we push an element, we increment this index and store it at that position. When we pop an element, we return its value and decrement this index.
The array implementation has constant-time complexity for push and pop operations but requires predefining its size.
Linked List Implementation
In linked list implementation, each node holds a value and a reference to the next node. The top of the stack is represented by the first node in this linked list.
When we push an element, we create a new node and make it the new top by updating the reference. When we pop an element, we return its value and update the top reference to point to the next node.
The linked list implementation allows for dynamic resizing but requires extra memory for maintaining references.
Common Use Cases
Stacks are commonly used in various algorithms and applications:
- Function Call Stack: Stacks are used by programming languages to manage function calls. Each function call is added to the stack, and when a function completes execution, it is removed from the stack.
- Expression Evaluation: Stacks can be used to evaluate arithmetic expressions by converting them into postfix or prefix notations and then applying stack operations.
- Backtracking Algorithms: Many backtracking algorithms use stacks to keep track of possible choices and revert back when necessary.
In conclusion, stacks are an essential data structure with various applications in computer science. Understanding how they work and their common use cases can greatly enhance your problem-solving skills.