stack.py

class Node:
   def __init__(self, value):
      self.value = value
      self.next = None
 
class Stack:
    
   # Initializing a stack.
   # Use a dummy node, which is
   # easier for handling edge cases.
   def __init__(self):
      self.head = Node("head")
      self.size = 0
 
   # String representation of the stack
   def __str__(self):
      cur = self.head.next
      out = ""
      while cur:
         out += str(cur.value) + "->"
         cur = cur.next
      return out[:-3]  
 
   # Get the current size of the stack
   def getSize(self):
      return self.size
    
   # Check if the stack is empty
   def isEmpty(self):
      return self.size == 0
    
   # Get the top item of the stack
   def peek(self):
       
      # Sanitary check to see if we
      # are peeking an empty stack.
      if self.isEmpty():
         raise Exception("Peeking from an empty stack")
      return self.head.next.value
 
   # Push a value into the stack.
   def push(self, value):
      node = Node(value)
      node.next = self.head.next
      self.head.next = node
      self.size += 1
      
   # Remove a value from the stack and return.
   def pop(self):
      if self.isEmpty():
         raise Exception("Popping from an empty stack")
      remove = self.head.next
      self.head.next = self.head.next.next
      self.size -= 1
      return remove.value