doublylinkedlist.py

class Node(object):
    def __init__(self, data, next, prev):
        self.data = data
        self.next = next
        self.prev = prev


class LinkedList(object):
    def __init__(self):
        self.head = None
        self.tail = None
        self.count = 0

    def add(self, data):
        node = Node(data, None, None)
        if self.head is None:
            self.head = node
            self.tail = node
        else:
            self.head.prev = node
            node.next = self.head
            self.head = node
        self.count += 1

    def pop_head(self):
        if not self.head:
            raise Exception('You cannot pop_head() when the LL is empty.')

        popped_value = self.head.data
        if self.head is self.tail:
            self.head.next = None
            self.head.prev = None
            self.head = None
            self.count -= 1
            return popped_value
        elif self.head.next:
            self.head.next.prev = None
            self.head = self.head.next
            self.count -= 1
            return popped_value
        else:
            raise Exception("Unable to pop linked list head.")

    def pop_tail(self):
        if not self.tail:
            raise Exception('You cannot pop_head() when the LL is empty.')

        popped_value = self.tail.data
        if self.tail is self.head:
            self.tail.prev = None
            self.tail.next = None
            self.tail = None
            self.count -= 1
            return popped_value
        elif self.tail.prev:
            self.tail.prev.next = None
            self.tail = self.tail.prev
            self.count -= 1
            return popped_value
        else:
            raise Exception("Unable to pop linked list tail.")

    def peek_head(self):
        if self.head:
            return self.head.data
        else:
            raise Exception('No node to pop')

    def peek_tail(self):
        if self.tail:
            return self.tail.data
        else:
            raise Exception('No node to pop')

    def size(self):
        return self.count

    # def has_cycle(self):
    #     head = self.head
    #     count = self.count
    #     while count:
    #         head = head.next
    #         count -= 1
    #     return head is not None

    # def circular(self):
    #     fast = self.head
    #     slow = self.head
    #     while fast and fast.next:
    #         slow = slow.next
    #         fast = fast.next.next
    #         if fast == slow:
    #             return True
    #     return False

    def remove_node(self, node_to_remove):
        if not self.head:
            raise Exception()
        if node_to_remove == self.head:
            self.pop_head()
            return
        elif node_to_remove == self.tail:
            self.pop_tail()
            return

        node_to_remove.prev.next = node_to_remove.next
        node_to_remove.next.prev = node_to_remove.prev
        self.count -= 1
        return


ll = LinkedList()
ll.add(1)
ll.add(2)
ll.add(3)
ll.add(4)
ll.add(5)
ll.add(6)
ll.add(7)
# ll.add(8)
# ll.size()
# ll.peek()
# ll.head.next.next.next.next.next = ll.head.next
# ll.head.next.next.next.next.next.next.next.next = ll.head.next.next
# ll.pop()
# ll.is_valid()