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Quick Summary
Reconstruct a tree structure from a flat list where each element defines a node and references its children by array index.
Identify the unique root by tracking which node index never appears as a child reference.
Execute an iterative preorder traversal to output node key-value pairs in the specified visitation order.
What This Tests
Tree reconstruction from adjacency-style arrays
Root detection using index tracking or set operations
Iterative depth-first traversal implementation
Array indexing and state management without explicit object graphs
Common Patterns
Graph Traversal
DFS/BFS
Hashing
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