155. Min Stack

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.

Solution: Two Stacks

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

    def push(self, x):
        """
        :type x: int
        :rtype: void
        """
        self.stack.append(x)
        if self.min_stack:
            self.min_stack.append(min(self.min_stack[-1], x))
        else:
            self.min_stack.append(x)

    def pop(self):
        """
        :rtype: void
        """
        if self.stack:
            self.min_stack.pop()
            return self.stack.pop()

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

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