Leetcode: Min Stack

Posted on January 10, 2018September 10, 2019 by braindenny

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

Similar Problems:

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.

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

## Blog link: https://code.dennyzhang.com/min-stack
## Basic Ideas: List with elements of (value, min_value)
##    -2 0 -3  4
##    linked list:
##     (-2, -2), (0, -2), (-3, -3), (4, -3)
##
##    length: element count
## Complexity:  Time: O(1), Space: O(n)
class MinStack(object):

    def __init__(self):
        """
        initialize your data structure here.
        """
        self.length = 0
        self.min_value = None
        self.values = []


    def push(self, x):
        """
        :type x: int
        :rtype: void
        """
        if self.length == 0:
            element = (x, x)
            self.min_value = x
        else:
            if x > self.min_value:
                element = (x, self.min_value)
            else:
                self.min_value = x
                element = (x, x)
        self.values.append(element)
        self.length += 1

    def pop(self):
        """
        :rtype: void
        """
        if self.length == 0:
            return

        if self.length == 1:
            self.min_value = None
        else:
            (x, previous_min_value) = self.values[-2]
            self.min_value = previous_min_value

        self.values.pop()
        self.length -= 1

    def top(self):
        """
        :rtype: int
        """
        if len(self.values) == 0:
            return None
        else:
            (x, _min_value) = self.values[-1]
            return x

    def getMin(self):
        """
        :rtype: int
        """
        return self.min_value

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