DAY 3 — Stacks

2 min readSep 20, 2020

It’s a series of revising DS algo. Check this link for more details .

A stack is an ordered list in which insertion and deletion a re done al one end, called top. The last element inserted is the first one to be deleted. Hence, it is called the Last in First out (LI PO) or First in Last out (FI LO) list.

Applications

Stack is a wisely used data structure , some of them are below :

Balancing of symbols
lnfix-to-postfix conversion Evaluation of postfix expression
Implementing functions calls (including recursion)
Finding of s pans (finding spans in stock markets, refer to Problems section)
Page-visited history in a Web browser [Back Buttons]
Undo sequence in a text editor
Matching Tags in HTML and XML

We use stack to implement in graphs or tree travel also .

Implementing Stack

We can Implement stack using Array
We can Implement stack using Dynamic Array (List)
We can Implement stack using Linked List

1. Implementing Stack by Static Array in Python

class Stack():
    def __init__(self,limit = 10):
        self.stack = [] 
        self.limit = limit
              
    def isEmpty(self):
        return len(self.stack) <= 0 
        
    def push(self,value):
        if len(self.stack) >= self.limit:
            print("Stack is Overflow")
        else:
            self.stack.append(value)
    
    def pop(self):
        if len(self.stack) <= 0:
            print("Stack is Underflow")
            return 
        
        else:
            return self.stack.pop()
            
    def peek(self):
        if len(self.stack) <= 0:
            print("Stack Underflow")
            return 
        else:
            return self.stack[-1]
            
    def size(self):
        return len(self.stack)myStack = Stack()
myStack.push(3)
print(myStack.peek())

2. Implementing Stack by Dynamic Array in Python

class Stack:
    def __init__(self):
        self.stack = []
        
    def isEmpty(self):
        return self.stack == []
  
    def push(self,ele):
        self.stack.append(ele)
        
    def pop(self):
        if not self.isEmpty():
            return self.stack.pop()
        else:
            return -1
            
    def peek(self):
         if not self.isEmpty():
             return self.stack[-1]
          else:
               return -1

3. Implementing Stack by Linked List in Python

class Node: 
 def __init__(self,data): 
    self.data = data 
      self.next = None
 
class Stack:def __init__(self): 
  self.head = None
 
 def isempty(self): 
       if self.head == None: 
   return True
  else: 
   return False
 
 def push(self,data): 
  
  if self.head == None: 
   self.head=Node(data) 
   
  else: 
   newnode = Node(data) 
   newnode.next = self.head 
   self.head = newnode 
 
 def pop(self): 
  
  if self.isempty(): 
      return None
   
  else: 
   poppednode = self.head 
   self.head = self.head.next
     poppednode.next = None
   return poppednode.data 
 
 def peek(self): 
  
  if self.isempty(): 
   return None
   
  else: 
   return self.head.data 
 
  def display(self): 
  
  iternode = self.head 
  if self.isempty(): 
   print("Stack Underflow") 
  
  else: 
   
   while(iternode != None): 
    
    print(iternode.data,"->",end = " ") 
    iternode = iternode.next
   returnmyStack = Stack()myStack.push(1)
print(myStack.peek())

Sources

My LinkedIn — linkedin.com/in/my-pro-file

To become a part of this series checkout this link .