Union-find data structure for Nodes.
All operations have an accumulated worst-case complexity of O(a(n)), where a(n) is the inverse of
the Ackermann function A(n,n).
Get a representative element of the equivalence set of a node.
This function returns the same representative element for all members of the same equivalence
set, i.e., find(a) == find(b) if and only if a and b are in the same set.