1993. Operations On Tree - Explanation

class LockingTree:

    def __init__(self, parent: List[int]):
        self.parent = parent
        self.child = [[] for _ in range(len(parent))]
        self.locked = [0] * len(parent)
        for node in range(1, len(parent)):
            self.child[parent[node]].append(node)

    def lock(self, num: int, user: int) -> bool:
        if self.locked[num]:
            return False
        self.locked[num] = user
        return True

    def unlock(self, num: int, user: int) -> bool:
        if self.locked[num] != user:
            return False
        self.locked[num] = 0
        return True

    def upgrade(self, num: int, user: int) -> bool:
        node = num
        while node != -1:
            if self.locked[node]:
                return False
            node = self.parent[node]

        def dfs(node):
            lockedCount = 0
            if self.locked[node]:
                lockedCount += 1
                self.locked[node] = 0

            for nei in self.child[node]:
                lockedCount += dfs(nei)
            return lockedCount

        if dfs(num) > 0:
            self.locked[num] = user
            return True
        return False

class LockingTree:

    def __init__(self, parent: List[int]):
        self.parent = parent
        self.child = [[] for _ in range(len(parent))]
        self.locked = [0] * len(parent)
        for node in range(1, len(parent)):
            self.child[parent[node]].append(node)

    def lock(self, num: int, user: int) -> bool:
        if self.locked[num]:
            return False
        self.locked[num] = user
        return True

    def unlock(self, num: int, user: int) -> bool:
        if self.locked[num] != user:
            return False
        self.locked[num] = 0
        return True

    def upgrade(self, num: int, user: int) -> bool:
        node = num
        while node != -1:
            if self.locked[node]:
                return False
            node = self.parent[node]

        lockedCount, q = 0, deque([num])
        while q:
            node = q.popleft()
            if self.locked[node]:
                self.locked[node] = 0
                lockedCount += 1
            q.extend(self.child[node])

        if lockedCount:
            self.locked[num] = user
            return True
        return False

class LockingTree:

    def __init__(self, parent: List[int]):
        self.parent = parent
        self.child = [[] for _ in range(len(parent))]
        self.locked = [0] * len(parent)
        for node in range(1, len(parent)):
            self.child[parent[node]].append(node)

    def lock(self, num: int, user: int) -> bool:
        if self.locked[num]:
            return False
        self.locked[num] = user
        return True

    def unlock(self, num: int, user: int) -> bool:
        if self.locked[num] != user:
            return False
        self.locked[num] = 0
        return True

    def upgrade(self, num: int, user: int) -> bool:
        node = num
        while node != -1:
            if self.locked[node]:
                return False
            node = self.parent[node]

        lockedCount, stack = 0, [num]
        while stack:
            node = stack.pop()
            if self.locked[node]:
                self.locked[node] = 0
                lockedCount += 1
            stack.extend(self.child[node])

        if lockedCount:
            self.locked[num] = user
            return True
        return False