Min Stack

Problem Statement

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

Implement the MinStack class:

• MinStack() initializes the stack object.
• void push(int val) pushes the element val onto the stack.
• void pop() removes the element on the top of the stack.
• int top() gets the top element of the stack.
• int getMin() retrieves the minimum element in the stack.

You must implement a solution with O(1) time complexity for each function.

Leetcode link

Example 1:

Input
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

Output
[null,null,null,null,-3,null,0,-2]

Explanation
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); // return -3
minStack.pop();
minStack.top();    // return 0
minStack.getMin(); // return -2

Constraints:

• -2³¹ <= val <= 2³¹ - 1
• Methods pop, top and getMin operations will always be called on non-empty stacks.
• At most 3 * 104 calls will be made to push, pop, top, and getMin.

Code

Python Code

class MinStack(object):

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


    def push(self, x):
        """
        :type x: int
        :rtype: void
        """
        return self.stack.insert(0,x)


    def pop(self):
        """
        :rtype: void
        """
        temp = self.stack[0]
        del self.stack[0]
        return temp


    def top(self):
        """
        :rtype: int
        """
        return self.stack[0]


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