class QueueWithTwoStacks(object):

     def __init__(self):

         self._stack1 = []

         self._stack2 = []

     def appendTail(self,x):

         self._stack1.append(x)

     def deleteHead(self):

          if self._stack2:

              return self._stack2.pop()

          else:

              if self._stack1:

                 while self._stack1:

                     self._stack2.append(self._stack1.pop())

                 return self._stack2.pop()

              else:

                  return None

 class StackWithTwoQueues(object):

     def __init__(self):

         self._stack1 = []

         self._stack2 = []

     def push(self,x):

         if len(self._stack1) == 0:

             self._stack1.append(x)

         elif len(self._stack2) == 0:

             self._stack2.append(x)

         if len(self._stack2) == 1 and len(self._stack1) >= 1:

             while self._stack1:

                 self._stack2.append(self._stack1.pop(0))

         elif len(self._stack1) == 1 and len(self._stack2) > 1:

             while self._stack2:

                 self._stack1.append(self._stack2.pop(0))

     def pop(self):

         if self._stack1:

             return self._stack1.pop(0)

         elif self._stack2:

             return self._stack2.pop(0)

         else:

             return None