Solutions

# Use a second stack to keep track of minimum so far. Pop and push every time to the minstack too.
# stack

# Min stack: O(1) time, O(n) space

class MinStack:

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

    def push(self, val: int):
        self.stack.append(val)
        if not self.minstack:
            self.minstack.append(val)
        else:
            mini = min(self.minstack[-1], val)
            self.minstack.append(mini)

    def pop(self) -> None:
        self.stack.pop()
        self.minstack.pop()

    def top(self) -> int:
        return self.stack[-1]

    def get_min(self) -> int:
        return self.minstack[-1]