a41e2fd963
The basic conversion is a straightforward conversion of the linear recursion to a loop forwards and backwards propagation of the result. But this uses an optimization to avoid the need for extra space that would otherwise be necessary to store the stack of unfinished states as the function is not tail recursive. Observe that only non-root-nodes in cycles have a recursive call and that every such call overwrites their own node state. Thus we reuse the node state itself as temporary storage for the stack of unfinished states by inverting the links to a chain back to the previous state update. When we hit the root or end of the full explored chain we propagate the node state update backwards by following the chain until a node with a link to itself. |
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