Rebuild Binary Tree from Level Order

levelOrder is a compact breadth-first serialization of a binary tree. The first value is the root. For each non-null node removed from a queue, the next value (if present) is its left child and the following value (if present) is its right child. nullMarker means that child is absent and is never a real node value. Trailing absent children may be omitted.
Rebuild the tree and return its preorder serialization, writing nullMarker for every absent left or right child.

Function
levelOrderToPreorder(levelOrder: int[], nullMarker: int) → int[]

Examples
Example 1
levelOrder = [1,2,3,-1,4,-1,5]
nullMarker = -1
return = [1,2,-1,4,-1,-1,3,-1,5,-1,-1]
Node 1 has children 2 and 3; node 2 has only right child 4; node 3 has only right child 5. Preorder records each missing child as -1.

Constraints
1 <= levelOrder.length <= 100000.
The encoding is valid and completely consumed by the reconstruction rule.
Every real node value differs from nullMarker.
If the root equals nullMarker, it is the only input entry.

The trick is a queue plus a read index. Create the root, push it, then for each node you pop, consume the next value as its left child and the one after as its right. If the value equals nullMarker, leave that child empty. Otherwise create the node and enqueue it. Stop reading when the index passes the array end, because trailing absent children are omitted. That's the pitfall: don't assume every node gets two values. Check bounds before each read. Handle the root-is-marker case up front and return just [nullMarker]. With up to 100000 nodes, a skewed tree makes recursive preorder blow the stack in some languages, so use an explicit stack. Push right before left, and emit nullMarker whenever you hit an empty child. StealthCoder is your hedge if the live OA catches you mid-blank on the bounds logic.

If this hits your live OA and you blank, StealthCoder solves it in seconds, invisible to the proctor.