6. Find All Reachable Nodes in Tree

You're building a file system explorer that needs to list all files and folders accessible from a root directory.

Given the root of an N-ary tree, return all reachable node values in level-order (BFS).

Level-order means:

• Visit the root first
• Then visit nodes level by level
• Within the same level, preserve children order as provided in node.children

> Note: A DFS traversal can reach all nodes, but it will produce a different order for many test cases.

Requirements

1. Basic Functionality

• Find all nodes reachable from root
• Handle trees with any number of children
• Return values in level-order (BFS)
• Handle empty tree (return empty array)

Example Usage

// Tree structure:
//       1
//     / | \
//    2  3  4
//   /|  |
//  5 6  7

findReachableNodes(root);
// [1, 2, 3, 4, 5, 6, 7]
// BFS level-order output

Real-World Context

This problem models practical tree traversal scenarios:

• File system: List entries by folder depth
• Navigation trees: Build breadth-first menu structures
• Access control: Enumerate reachable resources from a root