Stack is a linear data structure which follows a particular order in which the operations are performed. The order may be LIFO(Last In First Out) or FILO(First In Last Out).

Mainly the following three basic operations are performed in the stack:

• Push: Adds an item in the stack. If the stack is full, then it is said to be an Overflow condition.
• Pop: Removes an item from the stack. The items are popped in the reversed order in which they are pushed. If the stack is empty, then it is said to be an Underflow condition.
• Peek or Top: Returns top element of stack.
• isEmpty: Returns true if stack is empty, else fals.
• How to understand a stack practically?
  There are many real life examples of stack. Consider the simple example of plates stacked over one another in canteen. The plate which is at the top is the first one to be removed, i.e. the plate which has been placed at the bottommost position remains in the stack for the longest period of time. So, it can be simply seen to follow LIFO/FILO order.Time Complexities of operations on stack:push(), pop(), esEmpty() and peek() all take O(1) time. We do not run any loop in any of these operations.Applications of stack:
  • Balancing of symbols
  • Infix to Postfix /Prefix conversion
  • Redo-undo features at many places like editors, photoshop.
  • Forward and backward feature in web browsers
  • Used in many algorithms like Tower of Hanoi, tree traversals, stock span problem, histogram problem.
  • Other applications can be Backtracking, Knight tour problem, rat in a maze, N queen problem and sudoku solver

  Implementation:
  There are two ways to implement a stack:

  • Using array
  • Using linked list

  Implementing Stack using Arrays

Python Programming:

# Python program for implementation of stack
 
# import maxsize from sys module 
# Used to return -infinite when stack is empty
from sys import maxsize
 
# Function to create a stack. It initializes size of stack as 0
def createStack():
    stack = []
    return stack
 
# Stack is empty when stack size is 0
def isEmpty(stack):
    return len(stack) == 0
 
# Function to add an item to stack. It increases size by 1
def push(stack, item):
    stack.append(item)
    print("pushed to stack " + item)
     
# Function to remove an item from stack. It decreases size by 1
def pop(stack):
    if (isEmpty(stack)):
        return str(-maxsize -1) #return minus infinite
     
    return stack.pop()
 
# Driver program to test above functions    
stack = createStack()
push(stack, str(10))
push(stack, str(20))
push(stack, str(30))
print(pop(stack) + " popped from stack")

Pros: Easy to implement. Memory is saved as pointers are not involved.
Cons: It is not dynamic. It doesn’t grow and shrink depending on needs at runtime.

Output:

10 pushed to stack
20 pushed to stack
30 pushed to stack
30 popped from stack
Top item is 20

       Implementing Stack using Linked List

Python Programming:

# Python program for linked list implementation of stack
 
# Class to represent a node
class StackNode:
 
    # Constructor to initialize a node
    def __init__(self, data):
        self.data = data 
        self.next = None
 
class Stack:
     
    # Constructor to initialize the root of linked list
    def __init__(self):
        self.root = None
 
    def isEmpty(self):
        return True if self.root is None else  False
 
    def push(self, data):
        newNode = StackNode(data)
        newNode.next = self.root 
        self.root = newNode
        print "%d pushed to stack" %(data)
     
    def pop(self):
        if (self.isEmpty()):
            return float("-inf")
        temp = self.root 
        self.root = self.root.next
        popped = temp.data
        return popped
     
    def peek(self):
        if self.isEmpty():
            return float("-inf")
        return self.root.data
 
# Driver program to test above class 
stack = Stack()
stack.push(10)        
stack.push(20)
stack.push(30)
 
print "%d popped from stack" %(stack.pop())
print "Top element is %d " %(stack.peek())
 
# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

Output:

10 pushed to stack
20 pushed to stack
30 pushed to stack
30 popped from stack
Top element is 20

Pros: The linked list implementation of stack can grow and shrink according to the needs at runtime.
Cons: Requires extra memory due to involvement of pointers.