dsu/kruskal_tree: tighten allocation to 2*n-1 (#242)

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.

Author: cameroncuster

library/dsu/kruskal_tree.hpp

@@ -1,12 +1,13 @@
 #pragma once
 //! https://mzhang2021.github.io/cp-blog/kruskal/
+//! requires n >= 1 (2*n-1 allocation underflows when n == 0)
 //! @time O(n log n)
 //! @space O(n)
 struct kr_tree {
   int id;
   vi p;
   vector<vi> g;
-  kr_tree(int n): id(n), p(2 * n, -1), g(2 * n) {}
+  kr_tree(int n): id(n), p(2 * n - 1, -1), g(2 * n - 1) {}
   int f(int u) { return p[u] < 0 ? u : p[u] = f(p[u]); }
   bool join(int u, int v) {
     if ((u = f(u)) == (v = f(v))) return 0;

library/strings/manacher/manacher.hpp

@@ -14,6 +14,7 @@
 //! string b a a b a
 //! center  1 3 5 7
 //!
+//! requires n >= 1 (2*n-1 allocation underflows when n == 0)
 //! @time O(n)
 //! @space O(n)
 vi manacher(const auto& s) {

library/strings/suffix_array/suffix_array.hpp

@@ -32,6 +32,7 @@
 //!   auto [sa1, sa_inv1, lcp1] = get_sa(s_vec, 100'001);
 //! @endcode
 //!
+//! requires n >= 1 (lcp size n-1 underflows when n == 0)
 //! @time O(nlogn + max_num)
 //! @space O(n + max_num)
 auto get_sa(const auto& s, int max_num) {

library/strings/suffix_array/suffix_array_short.hpp

@@ -8,6 +8,7 @@
 //!   auto [sa1, sa_inv1, lcp1] = sa_short(s_vec);
 //! @endcode
 //! about 2-3x slower than KACTL
+//! requires n >= 1 (lcp size n-1 underflows when n == 0)
 //! @time O(n * log^2(n))
 //! @space O(n)
 auto sa_short(const auto& s) {