Hard data structure and algorithm practice puzzle.
Given a routing trie where a node holds a next-hop id, or 0 when it carries no route, return how many places two sibling routes could collapse into their parent. Siblings that send to the same place say nothing their parent could not say for them, and merging them is what kept the global routing table from outgrowing its hardware after CIDR arrived in 1993. Two zeros are not a match: that is two nodes carrying no route, not two routes that agree.
Solve it three ways: snap the jigsaw pieces together, fill in the blanks, or type it from scratch against the tests. This one is free.
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