Identical Immutable Subtrees

An immutable tree library lets nodes be shared, so the structure is a forest in which a node may have several parents (a DAG with no cycles).

nodes describes every node as [node_id, value, children]:

• node_id is a unique string.
• value is a string label.
• children is an ordered list of child ids. Every child id appears in nodes, and the same child may appear more than once.

Two nodes are structurally identical when they have the same value, the same number of children, and their i-th children are structurally identical for every i. Child order matters, and node ids do not.

Return every group of two or more structurally identical nodes:

• each group is a list of node ids sorted in ascending string order,
• groups are sorted by their first id,
• return [] if no two nodes are identical.

Be careful: because nodes are shared, writing out a subtree in full can be exponentially long, and the structure can be very deep.