keep keys of different heaps updated when storing links to the same objects

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Designing one algorithm using Python I'm trying to maintain one invariant, but I don't know if that is even possible to maintain. It's part of an MST algorithm.

I have some "wanted nodes". They are wanted by one or more clusters, which are implemented as a list of nodes. If I get a node that is wanted by one cluster, it gets placed into that cluster. However, if more than one cluster want it, all those clusters get merged and then the node gets placed in the resulting cluster.

My goal
I am trying to get the biggest cluster of the list of "wanting clusters" in constant time, as if I had a max-heap and I could use the updated size of each cluster as the key.

What I am doing so far
The structure that I am using right now is a dict, where the keys are the nodes, and the values are lists with the clusters that want the node at the key. This way, if I get a node I can check in constant time if some cluster wants it, and in case there are, I loop through the list of clusters checking who is the biggest. Once I finish the loop, I merge the clusters by updating the information in all the smaller clusters. This way I get a total merging time of O(n logn), instead of O(n²).

Question
I was wondering if I could use something like a heap to store in my dict as the value, but I don't know how that heap would be updated with the current size of each cluster. Is it possible to do something like that by using pointers and possible other dict storing the size of each cluster?

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