Sanitize File Path

The function below builds a file path by concatenating a base directory with user-supplied input.

The bug: an attacker can pass "../../../etc/passwd" as the filename and read any file on the server — a classic path traversal (directory traversal) attack.

Your task: fix solve(basePath, userInput) so that:

• Valid filenames within the base directory return the full path.
• Any input that would escape basePath returns null.
• Use string operations only (no require('path') available in this sandbox).

solve('/uploads', 'photo.jpg')        // → '/uploads/photo.jpg'
solve('/uploads', 'sub/image.png')    // → '/uploads/sub/image.png'
solve('/uploads', '../etc/passwd')    // → null
solve('/uploads', '../../secret.key') // → null