剑指 Offer 30. 包含min函数的栈

思路：用辅助栈来存动态的最小值，pop时弹出栈顶的最小值即可。

Python：

class MinStack:

    def __init__(self):
        """
        initialize your data structure here.
        """
        self.stack=[]
        self.helpstack=[]

    def push(self, x: int) -> None:
        self.stack.append(x)
        if not self.helpstack or x<self.helpstack[-1]:
            self.helpstack.append(x)
        else:
            self.helpstack.append(self.helpstack[-1])

    def pop(self) -> None:
        self.stack.pop()
        self.helpstack.pop()

    def top(self) -> int:
        if self.stack:
            return self.stack[-1]
        else:
            return -1

    def min(self) -> int:
        if not self.helpstack:
            return -1
        else:
            return self.helpstack[-1]