数据结构——栈

class Stack:
    
    def __init__(self):
        self.stack = []
        
    def push(self,element):
        self.stack.append(element)
        
    def pop(self):
        return self.stack.pop()
    
    def get_top(self):
        if len(self.stack) > 0:
            return self.stack[-1]
        else:
            return None
    def is_empty(self):
        return len(self.stack) == 0