4-7 Python数据结构常考题之栈与队列

Posted 2021-07-21 WinvenChang

一、栈与队列

后进先出 VS 先进先出
1.熟练掌握用 python 的 list 或者 collections.deque()实现栈和队列
2.常考题：用栈实现队列
3.leetcode implement-queue-using-stacks

用栈实现队列图示分析：

代码实现：

# leetcode 第232号 用栈实现队列
from collections import deque


class Stack:
    def __init__(self):
        self.items = deque()

    def push(self, val):
        return self.items.append(val)

    def pop(self):
        return self.items.pop()

    def top(self):
        return self.items[-1]  # 返回栈顶值

    def empty(self):
        return len(self.items) == 0


class MyQueue:
    def __init__(self):
        self.s1 = Stack()
        self.s2 = Stack()

    def push(self, val):
        self.s1.push(val)

    def pop(self):
        if not self.s2.empty():
            return self.s2.pop()

        while not self.s1.empty():
            val = self.s1.pop()
            self.s2.push(val)
        return self.s2.pop()

    def peek(self):
        if not self.s2.empty():
            return self.s2.top()

        while not self.s1.empty():
            val = self.s1.pop()
            self.s2.push(val)
        return self.s2.top()

    def empty(self):
        return self.s1.empty() and self.s2.empty()


def test():
    q = MyQueue()
    q.push(1)
    q.push(2)
    q.push(3)
    print(q.pop())
    print(q.pop())
    print(q.pop())

test()