What is the stack?

In computer science, a stack is a linear data structure that follows a last-in, first-out (LIFO) principle. It is a collection of entities that are inserted and removed according to the LIFO principle. Stacks are used to storing data temporarily as part of computer memory.

In programming, a stack is often used to store data that is being used in a function call or as part of a programming algorithm. It is called a "stack" because the data is added to the top of the stack (pushed onto the stack) and removed from the top of the stack (popped off the stack). The data at the top of the stack is the data that was most recently added and the data at the bottom of the stack is the data that was added the longest time ago.

Some common operations on a stack include pushing an element onto the stack, popping an element off the stack, and checking the element at the top of the stack. Stacks are widely used in programming and are an important part of computer science.

Stacks are often implemented using an array or a linked list. In an array implementation, the stack is simply a section of the array that is being used as a stack, with the top of the stack represented by the end of the array. In a linked list implementation, each element of the stack is a node in the linked list, with the top of the stack represented by the head of the linked list.

Stacks are used in many different algorithms and data structures, including:

Evaluation of arithmetic expressions

Infix-to-postfix conversion

Function calls (to store the return address and to pass parameters)

Undo/redo functionality in text editors

Backtracking (e.g. in maze-solving algorithms)

The time complexity (i.e. the amount of time it takes) for stack operations is usually constant, O(1), because the top of the stack is always easily accessible. This makes stacks very efficient for certain types of problems.

In many programming languages, the stack is used as part of the memory management system, where it stores the local variables and intermediate values used by the program. The stack is usually managed by the compiler or interpreter, and the programmer does not have to worry about it directly.

Stacks can be implemented in a thread-safe manner, allowing multiple threads to safely use the stack concurrently. This can be done using a lock or a mutex (a mutual exclusion object) to synchronize access to the stack.

In some programming languages, the stack is used to store the call stack, which is a record of the function calls made by a program. The call stack is used to keep track of the point to which the program should return when a function call finishes.

Stacks can be used to implement a simple undo/redo mechanism in a program. For example, a text editor could store the changes made to a document in a stack, with the most recent change at the top. When the user wants to undo a change, the top change is popped off the stack and undone. When the user wants to redo a change, the change is pushed back onto the stack.

Stacks can be used to check for balanced parentheses in an arithmetic expression. In this case, the stack is used to store the parentheses as they are encountered in the expression. When a closing parenthesis is encountered, it is compared with the top element of the stack. If the top element is the corresponding opening parenthesis, the two parentheses are popped off the stack. If the parentheses are not balanced, the stack will not be empty at the end of the expression.

In computer science, the stack is often contrasted with the queue, which is a linear data structure that follows a first-in, first-out (FIFO) principle. While a stack allows for the insertion and deletion of elements only at one end (the top of the stack), a queue allows for the insertion of elements at one end (the back of the queue) and the deletion of elements at the other end (the front of the queue).

Stacks can be implemented in a variety of programming languages, including C, C++, Java, Python, and many others. In most programming languages, there is a built-in stack data structure or class that can be used to create stacks.

Stacks can be used to solve a variety of problems, including:

Checking whether a word is a palindrome (a word that is the same forwards and backward)

Finding the shortest path through a maze

Sorting a list of elements

Stacks can also be used to implement recursive function calls. In a recursive function, a function calls itself with a modified version of the original input. The stack is used to store the intermediate states of the function so that it can return to the correct point when the recursive call finishes.

One potential drawback of using a stack is that it has a limited amount of space, which is determined when the stack is created. If the stack becomes full and a new element needs to be pushed onto the stack, a stack overflow error will occur. This can be avoided by using a dynamically resizing stack, which automatically increases its size as needed.

In computer science, the stack is often used as an abstract data type with two major operations, namely, push and pop. The push operation adds an element to the stack, while the pop operation removes the top element from the stack. Other common operations on a stack include peek, which returns the value of the top element without removing it from the stack, and isEmpty, which returns a boolean value indicating whether the stack is empty.

Some variations of the stack data structure include the following:

A deque (double-ended queue), which is a stack that allows for the insertion and deletion of elements at both ends

A multi-stack, which is a stack that consists of multiple smaller stacks, each with its own fixed size

A priority stack is a stack in which elements have a priority associated with them and are popped off the stack in order of priority.

Stacks are considered a linear data structure because the elements in a stack are arranged in a linear fashion, with the first element added to the stack at the bottom and the last element added at the top.

In some programming languages, stacks are referred to as "last-in, first-out" (LIFO) lists. This terminology reflects the fact that the last element added to the stack (the element at the top of the stack) is the first element to be removed (popped off the stack).

Stacks can be used to reverse the order of a list of elements. This can be done by pushing the elements of the list onto a stack one by one, and then popping the elements off the stack and adding them to a new list. The resulting list will be the reverse of the original list.

Stacks can also be used to check whether an arithmetic expression is syntactically correct. In this case, the stack is used to store the operators and parentheses as they are encountered in the expression. If the expression is syntactically correct, the stack will be empty at the end of the expression.