class Node(object):
    def __init__(self, obj=None):
        self.obj = obj
        self.nxt = None

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


class SingleLinkedList(object):
    def __init__(self):
        self.head = None
        self.tail = None

    def append(self, obj):
        new_obj = Node(obj)
        if self.is_empty():
            self.head = new_obj
            self.tail = new_obj
        else:
            self.tail.nxt = new_obj
            self.tail = new_obj

    #   удаляет все элементы, содержащие указанное значение
    def remove_all(self, value):
        found = False
        prev = None
        cur = self.head
        while cur is not None:
            if cur.obj == value:
                found = True
                if prev is None:
                    if self.tail == self.head:
                        self.head = None
                        self.tail = None
                        cur = None
                    else:
                        cur = cur.nxt
                        self.head = cur
                elif cur.nxt is None:
                    self.tail = prev
                    prev.nxt = None
                    cur = None
                else:
                    prev.nxt = cur.nxt
                    cur = cur.nxt
            else:
                prev = cur
                cur = cur.nxt
        if not found:
            raise ValueError('value is not in list')

    def __str__(self):
        if not self.is_empty():
            cur = self.head
            out = f'LinkedList[{cur}'
            while cur.nxt is not None:
                cur = cur.nxt
                out += ', ' + str(cur)
            return out + ']'
        else:
            return 'LinkedList[]'

    def is_empty(self):
        return self.head is None

    def __len__(self):
        l=0
        cur=self.head
        while cur is not None:
            l+=1
            cur=cur.nxt
        return l
    def insert(self,index,value):
        i=0
        prev=None
        cur=self.head
        if self.append(self,value):
            self.append(self,value)
        else:
            while i!=index:
                prev=cur
                cur=cur.nxt
                i+=1
            prev.nxt=value
            cur.nxt=cur
            cur=value
    def __contains__(self, value):
        found = False
        prev = None
        cur = self.head
        while cur is not None:
            if cur.obj == value:
                found = True
            else:
                prev.nxt = cur.nxt
                cur = cur.nxt
        return found

class Stack:
    def __init__(self):
        self.top = 0
        self.head = Node()

    def is_empty(self):
        return self.top == 0

    def push(self, item):
        oldHead = self.head
        self.head = Node()
        self.head.item = item
        self.head.link = oldHead
        self.top += 1

    def pop(self):

        item = self.head.item
        self.head = self.head.link
        self.top -= 1
        return item

    def topp(self):
        return self.head.item

    def size(self):
        return self.top

stack=Stack()
stack.push(10)
stack.push(20)
stack.push(30)
print(stack.topp())
print(stack.size())