牛客-剑指0ffer-包含min函数的栈

题目描述
定义栈的数据结构，请在该类型中实现一个能够得到栈中所含最小元素的min函数（时间复杂度应为O（1））。

# -*- coding:utf-8 -*-
class Solution:
    def __init__(self):
        self.main_stack = []
        self.assist_stack = []
        self._min = None
    def push(self, node):
        # write code here
        self.main_stack.append(node)
        if self._min == None:
            self._min = node
        else:
            if node < self._min:
                self._min = node
        self.assist_stack.append(self._min)
    def pop(self):
        # write code here
        if len(self.main_stack) == 0:
            raise Exception('no data')
        elif len(self.main_stack) == 1:
            self._min = None
            self.assist_stack.pop()
            return self.main_stack.pop()
        else:
            self.assist_stack.pop()
            self._min = self.assist_stack[-1]
            return self.main_stack.pop()
    def top(self):
        # write code here
        return self.main_stack[-1]
    def min(self):
        # write code here
        return self._min