A tree on nodes 1 to n had one extra edge added, so edges has length n and contains exactly one cycle. Return an edge you can remove to get a tree again; if several work, return the one that appears last in edges.
The highlighted line runs next. The pill shows the live values in the running call.
The data structures the code is building
Undirected edges; laid out by distance from node 1
V = n, E = number of edges. The highlighted row is the solution being traced.
Keyboard: ← → step, Space play or pause. Enter 3 to 9 edges like [[1,2],[2,3],[1,3]]; with n edges the nodes are 1 to n, and the graph must be a connected tree plus one extra edge.