dsu/kruskal_tree: tighten allocation to 2*n-1#242
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A Kruskal reconstruction tree over n leaves has at most 2*n-1 nodes, so allocate exactly that instead of 2*n. Document that n >= 1 is required, since 2*n-1 (and the lcp size n-1 in the suffix-array / manacher code) underflows to a huge size_t when n == 0.
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What
Tighten the
kr_treeallocation from2 * nto2 * n - 1, the exactupper bound on the number of nodes in a Kruskal reconstruction tree over
nleaves.Why
The old
2 * nsizing was one element too large. The tight2 * n - 1matches the actual node count.
Note on
n == 02 * n - 1underflows to a hugesize_twhenn == 0, so the librarynow documents
requires n >= 1at each site with this kind ofallocation. This is the same latent constraint that already existed for
the suffix-array
lcp(sizen - 1) and manacher (2 * n - 1) code, soa
//! requires n >= 1note was added there as well for discoverability.Files
library/dsu/kruskal_tree.hpp— code fix (2*n→2*n-1) + notelibrary/strings/manacher/manacher.hpp— notelibrary/strings/suffix_array/suffix_array.hpp— notelibrary/strings/suffix_array/suffix_array_short.hpp— note