155. Min Stack

Description:

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.

'''
[1] self.min)

idea::
using self.min keep current min value.
but, when top value is current min, if pop, have to find new current_min. it takes O(n). n==len(stack)

so, it is not correct.
'''
'''
[2] using pairs)

idea::
when get_min() method is called, there is a top. and if not pop the top, below values are fixed. use this fact.
so, when push(val), push([val, min_val]), min_value would be current_min or new pushed value.

code::
(below)

- T/C: all method O(1)
- S/C: O(n). for stack, n==pushed values
'''

class MinStack:

    def __init__(self):
        self.stack=[]

    def push(self, val: int) -> None:
        cur_min=self.getMin()
        if self.empty() or cur_min > val:
            self.stack.append([val,val])
        else:
            self.stack.append([val,cur_min])
        
    def pop(self) -> None:
        if not self.empty():
            self.stack.pop()

    def top(self) -> int:
        if not self.empty():
            return self.stack[-1][0]

    def getMin(self) -> int:
        if not self.empty():
            return self.stack[len(self.stack)-1][1]
        return None
    
    def empty(self) -> bool:
        return len(self.stack)==0


# Your MinStack object will be instantiated and called as such:
# obj = MinStack()
# obj.push(val)
# obj.pop()
# param_3 = obj.top()
# param_4 = obj.getMin()