1. class Stack:
    3.     def __init__(self):
    4.         self._items=[]
    6.     def size(self):
    7.         # 返回栈元素个数
    8.         return len(self._items)
    10.     def is_empty(self):
    11.         return self._items==[]
    13.     def push(self, item):
    14.         # 添加新元素到栈顶
    15.         self._items.append(item)
    17.     def pop(self):
    18.         # 弹出栈顶元素
    19.         return self._items.pop()
    21.     def peek(self):
    22.         # 返回栈顶元素
    23.         return self._items[self.size()-1]
    25. if __name__ == '__main__':
    26.     stack = Stack()
    27.     stack.push(2)
    28.     stack.push(3)
    29.     stack.push(4)
    30.     stack.push(5)
    31.     tmp = stack.pop()
    32.     print(tmp)
    33.     print(stack.peek())
    34.     print(stack.size())
    35.     print(stack.is_empty())