02_最小栈

class MinStack(object):

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

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

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

    def top(self):
        """
        :rtype: int
        """
        if self.nums:
            return self.nums[-1]
        return 0

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


# 上述方法的pop(0)方法，每次都要移动数组，而且min方法的耗时不是常数 用空间换时间
class MinStack2(object):

    # 辅助栈和数据栈不同步
    # 关键 1：辅助栈的元素空的时候，必须放入新进来的数
    # 关键 2：新来的数小于或者等于辅助栈栈顶元素的时候，才放入（特别注意这里等于要考虑进去）
    # 关键 3：出栈的时候，辅助栈的栈顶元素等于数据栈的栈顶元素，才出栈，即"出栈保持同步"就可以了

    def __init__(self):
        # 数据栈
        self.data = []
        # 辅助栈
        self.helper = []

    def push(self, x):
        self.data.append(x)
        # 关键 1 和关键 2
        if len(self.helper) == 0 or x <= self.helper[-1]:
            self.helper.append(x)

    def pop(self):
        # 关键 3：【注意】不论怎么样，数据栈都要 pop 出元素
        top = self.data.pop()

        if self.helper and top == self.helper[-1]:
            self.helper.pop()
        return top

    def top(self):
        if self.data:
            return self.data[-1]

    def getMin(self):
        if self.helper:
            return self.helper[-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()


# 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()