A stack is a linear data structure which uses the same principle, i.e., the elements in a stack are added and removed only from one end, which is called the TOP. Hence, a stack is called a LIFO (Last-In-First-Out) data structure, as the element that was inserted last is the first one to be taken out.

A stack supports three basic operations:

PUSH : The push operation is used to insert an element into the stack. The new element is added at the topmost position of the stack. However, before inserting the value, we must first check if TOP=MAX–1, because if that is the case, then the stack is full and no more insertions can be done. If an attempt is made to insert a value in a stack that is already full, an OVERFLOW message is printed.

Figure 2: Algorithm for Push element into stack

POP : The pop operation is used to delete the topmost element from the stack. However, before deleting the value, we must first check if TOP=NULL because if that is the case, then it means the stack is empty and no more deletions can be done. If an attempt is made to delete a value from a stack that is already empty, an UNDERFLOW message is printed.

Figure 3: Algorithm to pop element from stack

PEEK: Peek is an operation that returns the value of the topmost element of the stack without deleting it from the stack. However, the Peek operation first checks if the stack is empty, i.e., if TOP = NULL, then an appropriate message is printed, else value is returned.

Figure 4: Algorithm for Peek operation