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Fix Graphs.hasCycle missing cycles in undirected graphs. - #8801
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Since the switch from recursion to iteration in 63734b9, `subgraphHasCycle` pops a node and marks it `COMPLETE` as soon as it dequeues the node it was reached from (the A->B->A case that `canTraverseWithoutReusingEdge` rejects), so the node's remaining successors are never visited. In an undirected graph whose successor order puts the parent first, cycles reachable only through those successors are missed. Connected undirected graphs with a cycle are caught by the `numEdges >= nodes().size()` shortcut, so this shows up when the graph has more nodes than edges, e.g. a triangle plus an isolated node. Fixes #8796 RELNOTES=n/a PiperOrigin-RevId: 1009602021
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Fix
Graphs.hasCyclemissing cycles in undirected graphs.Since the switch from recursion to iteration in 63734b9,
subgraphHasCyclepops a node and marks itCOMPLETEas soon as it dequeues the node it was reached from (the A->B->A case thatcanTraverseWithoutReusingEdgerejects), so the node's remaining successors are never visited. In an undirected graph whose successor order puts the parent first, cycles reachable only through those successors are missed.Connected undirected graphs with a cycle are caught by the
numEdges >= nodes().size()shortcut, so this shows up when the graph has more nodes than edges, e.g. a triangle plus an isolated node.Fixes #8796
RELNOTES=n/a