面试题21：包含min函数的栈

【题目】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。
【思路】

引入辅助栈min_stack，保持栈顶为该栈最小值
【代码】

class Solution:
    def __init__(self):
        self.stack = []
        self.min_stack = []
    def push(self, node):
        # write code here
        self.stack.append(node)
        if len(self.min_stack)==0:
            self.min_stack.append(node)
        else:
            if node<self.min_stack[-1]:
                self.min_stack.append(node)
            else:
                self.min_stack.append(self.min_stack[-1])
    def pop(self):
        # write code here
        if len(self.stack) == 0:
            return 
        else:
            self.min_stack.pop()
            a = self.stack.pop()
            return a
    def top(self):
        # write code here
        if len(self.stack) == 0:
            return 
        return self.stack[-1]
    def min(self):
        # write code here
        if len(self.min_stack) == 0:
            return 
        return self.min_stack[-1]