Solution.py

class Node:
    def __init__(self, key='', cnt=0):
        self.prev = None
        self.next = None
        self.cnt = cnt
        self.keys = {key}

    def insert(self, node):
        node.prev = self
        node.next = self.next
        node.prev.next = node
        node.next.prev = node
        return node

    def remove(self):
        self.prev.next = self.next
        self.next.prev = self.prev


class AllOne:
    def __init__(self):
        self.root = Node()
        self.root.next = self.root
        self.root.prev = self.root
        self.nodes = {}

    def inc(self, key: str) -> None:
        root, nodes = self.root, self.nodes
        if key not in nodes:
            if root.next == root or root.next.cnt > 1:
                nodes[key] = root.insert(Node(key, 1))
            else:
                root.next.keys.add(key)
                nodes[key] = root.next
        else:
            curr = nodes[key]
            next = curr.next
            if next == root or next.cnt > curr.cnt + 1:
                nodes[key] = curr.insert(Node(key, curr.cnt + 1))
            else:
                next.keys.add(key)
                nodes[key] = next
            curr.keys.discard(key)
            if not curr.keys:
                curr.remove()

    def dec(self, key: str) -> None:
        root, nodes = self.root, self.nodes
        curr = nodes[key]
        if curr.cnt == 1:
            nodes.pop(key)
        else:
            prev = curr.prev
            if prev == root or prev.cnt < curr.cnt - 1:
                nodes[key] = prev.insert(Node(key, curr.cnt - 1))
            else:
                prev.keys.add(key)
                nodes[key] = prev
        curr.keys.discard(key)
        if not curr.keys:
            curr.remove()

    def getMaxKey(self) -> str:
        return next(iter(self.root.prev.keys))

    def getMinKey(self) -> str:
        return next(iter(self.root.next.keys))


# Your AllOne object will be instantiated and called as such:
# obj = AllOne()
# obj.inc(key)
# obj.dec(key)
# param_3 = obj.getMaxKey()
# param_4 = obj.getMinKey()