Define Stack

A Stack is an ordered list in which all insertions (Push operation) and deletion (Pop operation) are made at one end, called the top. The topmost element is pointed by top. The top is initialized to -1 when the stack is created that is when the stack is empty. In a stack S = (a1,an), a1 is the bottom most element and element a is on top of element ai-1. Stack is also referred as Last In First Out (LIFO) list.