Min-Tracking Stack

medium

A warehouse stacks crates, each stamped with a weight. The floor manager keeps asking, mid-shift: "what's the lightest crate currently in the stack?" — and wants the answer instantly, every time.

Design a stack supporting all of these in O(1):

push x — put crate of weight x on top
pop — remove the top crate
top — report the top crate's weight
get_min — report the lightest weight currently in the stack

Operations format. Your function receives one argument ops: a list of operations, each itself a list — ["push", x], ["pop"], ["top"], or ["get_min"]. Process them in order and return a list with one entry per operation: the returned value for top / get_min, and None for push / pop. All pop/top/get_min calls are guaranteed to occur on a non-empty stack.

Example 1

in ops = [["push", -2], ["push", 0], ["push", -3], ["get_min"], ["pop"], ["top"], ["get_min"]]

out [None, None, None, -3, None, 0, -2]

After popping -3, the lightest remaining crate is -2.

Example 2

in ops = [["push", 5], ["get_min"], ["push", 3], ["get_min"], ["pop"], ["get_min"]]

out [None, 5, None, 3, None, 5]

Constraints

· 1 <= len(ops) <= 10^4
· -10^9 <= x <= 10^9
· pop, top and get_min are never called on an empty stack
· every operation must run in O(1)