//Useful for background tasks, uploading resources, print/task 
 
//Can be impleneted with array (use push/shift or unshift/pop) or queue class 
//Again any time you use shift/unshift, you have O(N)
//FIFO 

class Node: 
  def __init__(self, value, next=None): 
    self.value = value 
    self.next = next 
  
class Queue: 
  def __init__(self, first=None, last=None, size=0): 
    self.first = first 
    self.last = last 
    self.size = size 
  
  #add to end of queue (similar to append or push) and returns size 
  def enqueue(self, value): 
    new_node = Node(value) 
    
    if self.first == None: 
      self.first = new_node 
      self.last = new_node 
    else: 
      self.last.next = new_node
      self.last = new_node 
    
    self.size += 1
    return self.size 
  
  #remove from beginning of queue 
  def dequeue(self): 
    if self.first == None: 
      return None 
    
    temp = self.first 
    if self.first == self.last: 
      self.last = None 
    #set 1st property to be original first's next 
    self.first = self.first.next 
    self.size -= 1 
    
    return temp.value 
    
  #useful to visualize any other function works 
  def print_queue_to_list(self): 
    result = []
    current = self.first 
    while current: 
      result.append(current.value)
      current = current.next 
    print(result)

s = Queue() 
s.enqueue(1) 
s.enqueue(2)
s.enqueue(3)
s.print_queue_to_list() >> [1, 2, 3]
s.dequeue() 
s.print_queue_to_list() >> [2, 3]

// Big O: 
// Insertion and Removal: O(1) **this is mainly what stacks are good for 
// Access and Search: O(N) 
//Recall with array implenetation, insertion and removal is not constant