Add Balkan OI 2018 problem link to hld.md

Hostek · web-flow · commit 97dd3184249b · 2025-11-19T13:44:51.000+01:00
diff --git a/src/graph/hld.md b/src/graph/hld.md
@@ -190,3 +190,4 @@ int query(int a, int b) {
 - [Codeforces - Tree Queries](https://codeforces.com/contest/1254/problem/D)
 - [Codeforces - Tree or not Tree](https://codeforces.com/contest/117/problem/E)
 - [Codeforces - The Tree](https://codeforces.com/contest/1017/problem/G)
+- [Balkan OI 2018 - Min-max tree](https://oj.uz/problem/view/BOI18_minmaxtree)
