diff --git a/library/dsu/kruskal_tree.hpp b/library/dsu/kruskal_tree.hpp
index e854110c..55f49f99 100644
--- a/library/dsu/kruskal_tree.hpp
+++ b/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;
diff --git a/library/strings/manacher/manacher.hpp b/library/strings/manacher/manacher.hpp
index fa1f7514..10ea292c 100644
--- a/library/strings/manacher/manacher.hpp
+++ b/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) {
diff --git a/library/strings/suffix_array/suffix_array.hpp b/library/strings/suffix_array/suffix_array.hpp
index 1b8ed537..93b4b67e 100644
--- a/library/strings/suffix_array/suffix_array.hpp
+++ b/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) {
diff --git a/library/strings/suffix_array/suffix_array_short.hpp b/library/strings/suffix_array/suffix_array_short.hpp
index ea383be6..942c636c 100644
--- a/library/strings/suffix_array/suffix_array_short.hpp
+++ b/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) {