class Stack:
    def __init__(self, items = []) :
        self.items = items

    def push(self, item) :
        if isinstance(item, int) :
            self.items = [item] + self.items
        else :
            raise TypeError
        
    def __str__(self) :
        return str(self.items)

    def pop(self) :
        if self.isEmpty() :
            raise IndexError
        else :
            self.items = self.items[1:]

    def isEmpty(self) :
        return (self.items == [])
    
    def top(self) :
        if self.isEmpty() :
            raise IndexError
        else :
            return self.items[0]
    
s = Stack([3,4])
s.push(1)
s

class Queue2:
    def __init__(self, items = []) :
        self.items = items

    def push(self, item) :
        if isinstance(item, int) :
            self.items = [item] + self.items
        else :
            raise TypeError
        
    def __str__(self) :
        return str(self.items)

    def pop(self) :
        if self.isEmpty() :
            raise IndexError
        else :
            self.items = self.items[:-1]

    def isEmpty(self) :
        return (self.items == [])
    
    def top(self) :
        if self.isEmpty() :
            raise IndexError
        else :
            return self.items[-1]

q = Queue2()
q.push(8)
q.push(10)

'''
Trees!
'''
class Node :
    def __init__ (self, cargo = None) :
        self.cargo = cargo
        self.left = None
        self.right = None

    def __str__ (self) :
        return str(self.cargo)

def print_tree(tree) :
    if tree<>None :
        print_tree(tree.left)
        print tree.cargo
        print_tree(tree.right)
        
'''
def head(l) :
    return l[0]

    
def tail(l) :
    return l[1:]

class Node :
    def __init__ (self, cargo = None, next = None) :
        self.cargo = cargo
        self.next = next

    def __str__ (self) :
        return str(self.cargo)
    
def print_ll(linked_list) :
    print(linked_list)
    if linked_list.next <> None :
        print_ll(linked_list.next)
        
node1 = Node("test")
node2 = Node(3)
node2.next = node1
node3 = Node([1,2,3], node2)
node0 = Node("at the back")
node1.next = node0

def nth_element(linked_list, num) :
    if num==0 :
        return linked_list
    else :
        return nth_element(linked_list.next, num-1)
'''