[LC] 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.stack = []

    def push(self, x: int) -> None:
        if self.stack:
            self.stack.append((x, min(x, self.stack[-1][1])))
        else:
            self.stack.append((x, x))

    def pop(self) -> None:
        if not self.stack:
            return None
        return self.stack.pop()

    def top(self) -> int:
        if not self.stack:
            return None
        return self.stack[-1][0]

    def getMin(self) -> int:
        if not self.stack:
            return None
        return self.stack[-1][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()