leetcode：232. 用栈实现队列【双栈，一个辅助一个模拟队列】

分析

st1保持栈顶就是对顶，这样的话需要倒转两次
也就是st1每次接受一个新的x
先全部倒进去st2，然后st2再append x
最后再把st2全部倒入st1

ac code

class MyQueue:

    def __init__(self):
        self.st1 = [] # 从栈顶到栈底就是队列头到队列尾
        self.st2 = [] # 辅助栈


    def push(self, x: int) -> None:
        if not self.st1:
            self.st1.append(x)
        else:
            # 把st1倒过来
            while self.st1:
                self.st2.append(self.st1.pop())
            self.st2.append(x) # 补到最后
            # 再次回到st1，变成queue
            while self.st2:
                self.st1.append(self.st2.pop())
        #print(len(self.st1))


    def pop(self) -> int:
        #print(len(self.st1))
        return self.st1.pop()
        


    def peek(self) -> int:
        return self.st1[-1]


    def empty(self) -> bool:
        
        return len(self.st1) == 0



# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()

总结

双栈实现队列