设计一个支持 push，pop，top 操作，并能在常数时间内检索到最小元素的栈。
python3：
class MinStack:def __init__(self):"""initialize your data structure here."""self.val=[]self.index=-1def push(self, x: int) -> None:self.val.append(x)self.index+=1def pop(self) -> None:self.val.pop()self.index-=1def top(self) -> int:return self.val[self.index]def getMin(self) -> int:return min(self.val)# 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()