155. 最小栈

class MinStack:

    def __init__(self):
        """
        initialize your data structure here.
        """
        self.a=[]
        self.a_min=float('inf')

    def push(self, x: int) -> None:
        self.a.append(x)
        # 加入元素的时候检查最小值
        self.a_min=x if x<self.a_min else self.a_min

    def pop(self) -> None:
    # 删除元素的时候 重新检查最小值
        result = self.a[-1]
        del self.a[-1]
        if result==self.a_min:
            self.a_min=min(self.a) if self.a else float('inf')
        return result

    def top(self) -> int:
        return self.a[-1]

    def getMin(self) -> int:
        return self.a_min


# Your MinStack object will be instantiated and called as such:
# obj = MinStack()
# obj.push(x)
# obj.pop()
# param_3 = obj.top()
# param_4 = obj.getMin()