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Make tree iterators safe under concurrency

AppleSoftware Engineer · Technical Screen
HardSoftware EngineerTechnical ScreenSoftware Engineering Fundamentals

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This question tests the ability to design a thread-safe iterator for a binary search tree (BST) that remains consistent even if the underlying tree structure is modified during iteration.

Problem Statement

You are tasked with designing a thread-safe BSTIterator for an in-order traversal of a binary search tree. In a multi-threaded environment, other threads may perform insertions or deletions on the tree while an iterator is actively traversing it.

Define a class BSTIterator that supports the following operations:

  • next(): Returns the next value in the in-order traversal sequence.
  • hasNext(): Returns true if there are more elements to visit.

Requirements:

  1. The iterator must not crash if the tree is modified during traversal.
  2. You must decide on a 'snapshot' policy: either the iterator reflects the tree state as it existed when the iterator was created, or it reflects a 'live' view of the tree. Document your choice and the trade-offs.
  3. Assume the basic tree node has left, right, and value pointers.

Interface

class BSTNode:
    def __init__(self, value):
        self.value = value
        self.left = None
        self.right = None

class BSTIterator:
    def __init__(self, root: BSTNode):
        # Implementation
    
    def hasNext(self) -> bool:
        # Implementation

    def next(self) -> int:
        # Implementation

Considerations:

  • If you use locking (mutexes), how do you prevent deadlocks or excessive contention?
  • If you choose a lock-free or snapshot-based approach, how do you manage memory (e.g., if a node is deleted while the iterator holds a reference to it)?
  • Consider the worst-case time complexity of next() and space complexity of your iterator.

Hints

Hint 1

Consider copying the required nodes into a thread-safe queue or list upon initialization to provide a stable 'snapshot' of the tree.

Hint 2

If a snapshot is not feasible due to memory constraints, explore using parent pointers to allow the iterator to navigate upward if a node's subtree is reorganized.

Hint 3

Evaluate whether 'optimistic concurrency control' or 'versioning' (like a modification counter) could help detect if the tree structure has changed since the last `next()` call.

Follow-up questions

  • How would your design change if the tree was a balanced AVL or Red-Black tree where node rotations are frequent?
  • What are the performance implications of using a Reader-Writer lock versus a simple Mutex for the entire tree structure?
  • If you implemented a snapshot iterator, how would you handle the memory management of deleted nodes that might still be held by active iterators?