```#=======================================================================
# Author: Isai Damier
# Project: geekviewpoint
# Package: datastructure
#
# Description: A LinkedList is a data structure that allows access
#   to a collection of data using pointers/references. While an
#   array can also be defined as above, LinkedLists and arrays differ
#   in how they are stored in memory and in the operations they
#   allow. Unlike an array that must be stored in a block of memory,
#   the nodes of a LinkedList can be stored anywhere because each
#   node has a reference to the node that succeeds it. Because the
#   nodes are stored so loosely, inserting nodes into a LinkedList
#   is easy; whereas in an array, all the succeeding elements must
#   be shifted. Of course, insertion also means changing the size of
#   the array, which means creating the entire array anew.
#
#   Perhaps the greatest beauty of LinkedList is that it allows
#   accessing an entire sequence of nodes using only one variable:
#   a reference to the first node in the sequence.
#
#   Countless operations can be performed on LinkedLists. Following
#   are a few, ranging from the common to the very interesting.
#=======================================================================

#=====================================================================
# Statement:
#   Add element el after node x
#
# Time Complexity of Solution:
#   Best = O(1) Worst = O(n).
#
# Description:
#
# Technical Details: We simply need to find node x on the list
#   and then attach the new node containing el to said node, which
#   instead of having a traversal algorithm all the time, we treat the
#   possibility of x being the tail of the list as a special case
#   where the problem could be solved in O(1)
#=====================================================================
import collections

def __init__( self ):
self.head , self.tail = None, None

def addAfter( self, el, n ):
if None == self.tail or n.data == self.tail.data:
else:
while None != tmp and n.data != tmp.data:
tmp = tmp.next
if None != tmp:
tmp.next = Node( el, tmp.next )

class Node( object ):

def __init__( self, data, next = None ):
self.data = data
self.next = next
```
```import unittest
import random

class Test( unittest.TestCase ):
#=====================================================================