栈的最小值

请设计一个栈，除了常规栈支持的pop与push函数以外，还支持min函数，该函数返回栈元素中的最小值。执行push、pop和min操作的时间复杂度必须为O(1)。

示例：

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> 返回 -3.
minStack.pop();
minStack.top();      --> 返回 0.
minStack.getMin();   --> 返回 -2.

示例代码1：

class MinStack(object):

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


    def push(self, x):
        """
        :type x: int
        :rtype: None
        """
        self.stack.append(x)


    def pop(self):
        """
        :rtype: None
        """
        self.stack.pop()


    def top(self):
        """
        :rtype: int
        """
        return self.stack[-1]


    def getMin(self):
        """
        :rtype: int
        """
        return min(self.stack)



# 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()

示例代码2【【辅助栈】栈的最小值】：

class MinStack(object):

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


    def push(self, x):
        """
        :type x: int
        :rtype: None
        """
        if self.min_stack is not None:
            self.stack.append(x)
            self.min_stack.append(min(x, self.stack[-1]))
        else:
            self.stack.append(x)
            self.min_stack.append(x)


    def pop(self):
        """
        :rtype: None
        """
        self.stack.pop()
        self.min_stack.pop()


    def top(self):
        """
        :rtype: int
        """
        return self.stack[-1]


    def getMin(self):
        """
        :rtype: int
        """
        return self.min_stack[-1]



# 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()

本机电脑测试：

class MinStack(object):

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

    def push(self, x):
        """
        :type x: int
        :rtype: None
        """
        if self.min_stack is not None:
            self.stack.append(x)
            self.min_stack.append(min(x, self.stack[-1]))
        else:
            self.stack.append(x)
            self.min_stack.append(x)

    def pop(self):
        """
        :rtype: None
        """
        self.stack.pop()
        self.min_stack.pop()

    def top(self):
        """
        :rtype: int
        """
        return self.stack[-1]

    def getMin(self):
        """
        :rtype: int
        """
        return self.min_stack[-1]


# Your MinStack object will be instantiated and called as such:
x = -2
y = 0
z = -3
obj = MinStack()
obj.push(x)
obj.push(y)
obj.push(z)
print(obj.getMin())
obj.top()
print(obj.pop())
print(obj.getMin())

运行结果：