class Node:
    def __init__(self, element=None, index=0, predecessor=None, successor=None):
        self.element = element
        self.index = index
        self.predecessor = predecessor
        self.successor = successor

class LinkedListStack:
    def __init__(self):
        self.head = None
        self.size = 0

    def push(self, element):
        node = Node(element, self.size, None, None)
        if not self.head:
            self.head = node
        else:
            current = self.head
            while current.successor:
                current = current.successor
            current.successor = node
        self.size += 1

    def pop(self):
        if self.is_empty():
            return None
        else:
            node = self.head
            self.head = node.successor
            self.size -= 1
            return node.element

    def peek(self):
        if self.is_empty():
            return None
        else:
            return self.head.element

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

    def size(self):
        return self.size