15-155. Min Stack

题目描述：

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• getMin() -- Retrieve the minimum element in the stack.

Example:

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

代码：

class MinStack(object):

    def __init__(self):
        """
        initialize your data structure here.
        """
        self.stack = [] # 如果这个stack想在整个class中都可以使用，则必须要加上self.

    def push(self, x):
        """
        :type x: int
        :rtype: None
        """
        curMin = self.getMin()
        if curMin == None or x < curMin:
            curMin = x
        self.stack.append((x, curMin))

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

    def top(self):
        """
        :rtype: int
        """
        return self.stack[-1][0] if self.stack else None

    def getMin(self):
        """
        :rtype: int
        """
        return self.stack[-1][1] if self.stack else None


# 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()

分析：

1. python的栈、链表、树都是用list实现的。

2. 如果一个在class内部定义的变量想要在整个类中都可以使用，则这个变量的前面必须加上self. 。