堆栈和队列

栈 (Stack)是一种后进先出(last in first off，LIFO)的数据结构

队列(Queue)则是一种先进先出 (fisrt in first out，FIFO)的结构

栈

class Stack:
    def __init__(self):
        self.items = []

    def is_empty(self):
        return not self.items

    def push(self, item):
        """adds a new item to the top of the stack"""
        self.items.append(item)
    
    def pop(self):
        """
        removes the top item from the stack,
        popping an empty stack (list) will result in an error
        """
        if not self.is_empty():
            return self.items.pop()
        return "Pop from empty stack"

    def peek(self):
        """returns the top item from the stack but does not remove it"""
        if not self.is_empty():
            return self.items[len(self.items) - 1]
        return "Stack is Empty"

    def size(self):
        return len(self.items)

队列