The iterative approach in build_model failed to fill children arrays
correctly. A preorder traversal is not required and turned out to be the
culprit. Use an easier algorithm:
Add nodes from scaffold tree starting at index 0 (root) to the target
array whenever children are encountered. This ensures that children
are adjacent to each other. This complies with the recursive version.
Store only the scaffold index in numchildren field to prevent a direct
processing of these children, which would require a recursive solution.
This allows the algorithm to iterate through the target array from start
to end without jumping back and forth, converting on the fly.
Co-authored-by: Sebastian Pipping <sebastian@pipping.org>
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