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.

class MinStack:

    def __init__(self):
        """
        initialize your data structure here.
        """
        self.s = []
        self.min = []
    def push(self, x: int) -> None:
        self.s.append(x)
        if self.min == []:
            self.min.append(x)
        else:
            if x <= self.min[-1]:
                self.min.append(x)
        
    def pop(self) -> None:
        if self.s.pop() == self.min[-1]:
            self.min.pop()
        
    def top(self) -> int:
        return self.s[-1]

    def getMin(self) -> int:
        return self.min[-1]


# 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()