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