LeetCode刷题笔记（三）栈与队列

三. 栈与队列

python中的栈直接用list实现，队列用deque，需要导入外部包。

from collections import deque

155. 最小栈

题目：设计一个支持 push ，pop ，top 操作，并能在常数时间内检索到最小元素的栈。
思考：这道题考的是如何如何最快速度获取到最小元素。

class MinStack:
    def __init__(self):
        self.l = list()

    def push(self, val: int) -> None:
        self.l.append(val)

    def pop(self) -> None:
        self.l.pop()

    def top(self) -> int:
        return self.l[-1]

    def getMin(self) -> int:
        return min(self.l) # 不用辅助栈也能通过

class MinStack:
    def __init__(self):
        self.stack = []
        self.min_stack = [math.inf]

    def push(self, x: int) -> None:
        self.stack.append(x)
        self.min_stack.append(min(x, self.min_stack[-1]))

    def pop(self) -> None:
        self.stack.pop()
        self.min_stack.pop()

    def top(self) -> int:
        return self.stack[-1]

    def getMin(self) -> int:
        return self.min_stack[-1]

225. 用队列实现栈

题目：用队列实现栈
思考: 得用两个队列模拟栈

class MyStack:
    def __init__(self):
        self.queue1 = collections.deque()
        self.queue2 = collections.deque()

    def push(self, x: int) -> None:
        self.queue2.append(x)
        while self.queue1:
            self.queue2.append(self.queue1.popleft())
        self.queue1, self.queue2 = self.queue2, self.queue1

    def pop(self) -> int:
        return self.queue1.popleft()

    def top(self) -> int:
        return self.queue1[0]

    def empty(self) -> bool:
        return not self.queue1

232. 用栈实现队列

题目：用栈实现队列
思考: 得用两个栈模拟队列

class MyQueue:
    def __init__(self):
        self.a = []
        self.b = []

    def push(self, x: int) -> None:
        while self.b:
            self.a.append(self.b.pop())
        self.a.append(x)
        while self.a:
            self.b.append(self.a.pop())

    def pop(self) -> int:
        return self.b.pop()

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

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