题解 | #用两个栈实现队列#

class Solution:
    def __init__(self):
        self.stack1 = []
        self.stack2 = []
        
    def push(self, node):
        # write code here
        self.stack1.append(node)
    def pop(self):
        # return xx
        if not self.stack1 and not self.stack2: return None
        if self.stack2: return self.stack2.pop()
        else:
            while self.stack1: self.stack2.append(self.stack1.pop())
            return self.stack2.pop()