用两个栈实现队列与用两个队列实现栈（Python实现）

用两个栈实现队列：

 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

用两个队列实现栈：

 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