van Emde Boas Tree Structure (Universe = 16)
O(log log U) operations via recursive sqrt(U) clusters
For U = 16, the tree has sqrt(16) = 4 clusters, each of size 4. The summary tracks which clusters are non-empty. This recursion gives O(log log U) time for all operations.
Predecessor/successor only needs to check the current cluster or use the summary to jump to the next non-empty cluster. Each level halves the bit-length, giving log log U depth.