Trace simplified example with 4 items, 2 groups.
Items 0,1 in group 0. Items 2,3 in group 1. Dependencies: 2 depends on 0 (cross-group), 3 depends on 2 (same group).
Group graph: edge from group 0 → group 1 (because 2 depends on 0). Group order: [0, 1].
Group 0 items: topological sort of {0, 1}. Say [0, 1]. Group 1 items: 3 depends on 2. Order: [2, 3].
Final: [0, 1, 2, 3].
Build graphs: where E is total dependencies. Two topological sorts: total. Space: .