双堆栈实现队列

class Solution():
    def appendTail(self,A,B,element):
        A.append(element)
        return A

    def deleteHead(self,A,B):
        if B:
            return B.pop()
        else:
            while A:
                B.append(A.pop())
            return B.pop()




#test
A = []
B = []
s = Solution()
s.appendTail(A,B,'a')
s.appendTail(A,B,'b')
s.appendTail(A,B,'c')
s.deleteHead(A,B)
s.deleteHead(A,B)
s.appendTail(A,B,'d')
s.deleteHead(A,B)