用2个stack 完成 代码如下

class Queue:    # initialize your data structure here.    def __init__(self):        self.stackA = []        self.stackB = []    # @param x, an integer    # @return nothing    def push(self, x):        self.stackA.append(x)    # @return nothing    def pop(self):        while self.stackA != []:            self.stackB.append(self.stackA.pop())        self.stackB.pop()        while self.stackB != []:            self.stackA.append(self.stackB.pop())    # @return an integer    def peek(self):        while self.stackA != []:            self.stackB.append(self.stackA.pop())        x = self.stackB[-1]        while self.stackB != []:            self.stackA.append(self.stackB.pop())        return x    # @return an boolean    def empty(self):        return self.stackA == []