剑指Offer-- 包含min函数的栈

# -*- coding:utf-8 -*-
class Solution:
    res = []
    helper = []
    def push(self, node):
        self.res.append(node)
        if self.helper != []:
            if node < self.helper[-1]:      # 若是当前值比里边的最小值还小，那么就压入辅助栈
                self.helper.append(node)
            else:                           # 否则压入当前辅助栈的栈顶元素
                self.helper.append(self.helper[-1])
        if self.helper == []:
            self.helper.append(node)
    def pop(self):
        if self.res and self.helper:
            self.helper.pop()
            return self.res.pop()
    def top(self):
        # write code here
        if self.res:
            return self.res[-1]
    def min(self):
        # write code here
        if self.helper:
            return self.helper[-1]