Leetcode刷题笔记1 栈与队列part01

leetcode 232 用栈实现队列

此题考查的是对栈的掌握

class MyQueue:

    def __init__(self):
        self.stack_in = []
        self.stack_out = []

    def push(self, x: int) -> None:
        self.stack_in.append(x)

    def pop(self) -> int:
        if self.empty():
            return None
        if self.stack_out:
            return self.stack_out.pop()
        for i in range(len(self.stack_in)):
            self.stack_out.append(self.stack_in.pop())
        return self.stack_out.pop()

    def peek(self) -> int:
        ans = self.pop()
        self.stack_out.append(ans)
        return ans

    def empty(self) -> bool:
        return not(self.stack_in or self.stack_out)

leetcode 225 用队列实现栈

from collections import deque
class MyStack:

    def __init__(self):
        self.queue_in = deque()
        self.queue_out = deque()
    def push(self, x: int) -> None:
        self.queue_in.append(x)

    def pop(self) -> int:
        if self.empty():
            return None
        for i in range(len(self.queue_in) - 1):
            self.queue_out.append(self.queue_in.popleft())
        self.queue_in, self.queue_out = self.queue_out, self.queue_in
        return self.queue_out.popleft()

    def top(self) -> int:
        if self.empty():
            return None
        for i in range(len(self.queue_in) - 1):
            self.queue_out.append(self.queue_in.popleft())
        self.queue_in, self.queue_out = self.queue_out, self.queue_in
        tmp = self.queue_out.popleft()
        self.queue_in.append(tmp)
        return tmp

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

leetcode 20 有效的括号

栈的经典应用

class Solution:
    def isValid(self, s: str) -> bool:
        stack = []
        for item in s:
            if item == '(':
                stack.append(')')
            elif item == '[':
                stack.append(']')
            elif item == '{':
                stack.append('}')
            elif not stack or stack[-1] != item:
                return False
            else:
                stack.pop()
        return True if not stack else False