Lecture Notes on 21 Oct 2022

* Evaluate the following Reverse Polish expressions:

4 8 7 + 2 * 3 1 - * +
3 4 + 5 *
3 4 5 + *
7 4 + 3 - 2 5 * /


class Stack (object):
  def __init__ (self):
    self.stack = []

  # add an item to the top of the stack
  def push (self, item):
    self.stack.append (item)

  # remove an item from the top of the stack
  def pop (self):
    return self.stack.pop()

  # check the item on the top of the stack
  def peek (self):
    return self.stack[-1]

  # check if stack is empty
  def is_empty (self):
    return (len(self.stack) == 0)

  # return the number of elements in the stack
  def size (self):
    return (len (self.stack))



class Queue (object):
  def __init__ (self):
    self.queue = []

  # add an item to the end of the queue
  def enqueue (self, item):
    self.queue.append (item)

  # remove an item from the beginning of the queue
  def dequeue (self):
    return self.queue.pop(0)

  # check if the queue is empty
  def is_empty (self):
    return (len (self.queue) == 0)

  # return the number of elements in the queue
  def size (self):
    return (len (self.queue()))