Top View of a Binary Tree

You are given a binary tree serialized in level order as levelOrder. Each entry is either an integer written as a string or "null".
Assign horizontal distance 0 to the root. A left child has its parent's distance minus 1, and a right child has its parent's distance plus 1. For each horizontal distance, the first node reached in level order is visible from above.
Return the visible node values from the smallest horizontal distance to the largest. Return an empty array for an empty tree.

Function
topView(levelOrder: String[]) → int[]

Examples
Example 1
levelOrder = ["1","2","3","null","4","null","5"]
return = [2,1,3,5]
The first visible nodes at horizontal distances -1, 0, 1, and 2 are 2, 1, 3, and 5.
Example 2
levelOrder = ["10","5","15","2","7","12","20"]
return = [2,5,10,15,20]
The topmost values are read from the far-left horizontal distance through the far-right distance.
Example 3
levelOrder = []
return = []
An empty tree has no visible nodes.

Constraints
0 <= levelOrder.length <= 10^5
Every non-null entry is a valid 32-bit signed integer.
The serialization describes one valid binary tree in level order.

The trick: do a BFS and track a horizontal distance for each node. Root is 0, left child is minus 1, right child is plus 1. The first time you see a distance, store that node's value. Later nodes at the same distance are hidden. Because BFS goes level by level, first seen means topmost. Track min and max distance, then read the map from min to max. That avoids a sort. The common pitfall is using DFS, which can record a deeper node before a shallower one at the same distance. The second pitfall is the parsing. Use a queue of nodes and an index into the array. For each dequeued node, consume the next two entries as left and right, skipping "null". Handle the empty array early. With 10^5 entries, keep it O(n) and iterative. If the parsing or ordering trips you up live, StealthCoder is the hedge.

Drill it cold or hedge it with StealthCoder. Either way, don't walk into the OA hoping you remember the trick.