class UserList:
    """ A user defined singly linked list"""
    def __init__(self):
        self.head = None
        self.tail = None


    def __str__(self):
        """ String function """
        s = '['
        h = self.head
        while h != None:
            s += str(h.data)
            h = h.next
            if h != None:
                s += ', '
        s += ']'
        return s
    
    def insert_after(self, node):
        """ Insert a node with data at the end of the list
        """
        if self.is_empty():   # the node will be the first one in list
            self.head = node
            self.tail = node
        else:                 # insert after the current tail
            self.tail.next = node
            self.tail = node

    def insert_before(self, node):
        """ Insert a node with data before the head of the list
        """
        if self.is_empty():   # the node will be the first one in list
            self.head = node
            self.tail = node
        else:                 # insert before the current head
            node.next = self.head
            self.head = node

    def remove_node(self, target):
        """ Remove the node containing the target
        """
        prev = None
        cur  = self.head
        while cur != None and cur.data != target:
            prev = cur
            cur  = cur.next

        if cur != None:    # found it and remove it
            if cur == self.head:
                self.head = cur.next  # head is removed, reset head
            else:
                prev.next = cur.next  # remove a middle node
            
    def is_empty(self):
        """ Return True if the list is empty, False otherwise.
        """
        return self.head == None
            
class ListNode:
    """ The node class for list notes"""
    def __init__(self, data):
        self.data = data
        self.next = None