MinStack.py

# 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.

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

    def push(self, x):
        """
        :type x: int
        :rtype: nothing
        """
        self.stack.append(x)
        if self.min == []: self.min.append(x)
        else: self.min.append(min(self.min[-1], x))

    def pop(self):
        """
        :rtype: nothing
        """
        self.stack.pop()
        self.min.pop()

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

    def getMin(self):
        """
        :rtype: int
        """
        if self.min == []: return
        return self.min[-1]