剑指Offer(二十)：包含min函数的栈

解法1：对于python来讲，栈就是用list实现的！

class Solution:
    def __init__(self):
        self.stack = []
        self.assist = []
    def push(self,node):
        min = self.min()
        if not min or node < min:
            self.assist.append(node)
        else:
            self.assist.append(min)
        self.stack.append(node)
    def pop(self):
        if self.stack:
            self.assist.pop()
            return self.stack.pop()
    def top(self):
        if self.stack:
            return self.stack[-1]
    def min(self):
        if self.assist:
            return self.assist[-1]