From 975b2e396b73ca9e8c6309138335019ffa35bda5 Mon Sep 17 00:00:00 2001
From: prozolic <42107886+prozolic@users.noreply.github.com>
Date: Wed, 1 Apr 2026 23:50:31 +0900
Subject: [PATCH] Improve SortedSet<T>.TreeSubSet.Clear

Specify `Count` as the initial capacity of `List<T>` to avoid
unnecessary heap reallocations while collecting items via
`BreadthFirstTreeWalk`.

Replace the `while` loop that called `RemoveAt(Count - 1)` after each
removal with a simple reverse `for` loop over the list by index.
---
 .../System/Collections/Generic/SortedSet.TreeSubSet.cs    | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.TreeSubSet.cs b/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.TreeSubSet.cs
index f6c47050858cd8..e3ae983bdd7791 100644
--- a/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.TreeSubSet.cs
+++ b/src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.TreeSubSet.cs
@@ -96,12 +96,12 @@ public override void Clear()
                     return;
                 }
 
-                List<T> toRemove = new List<T>();
+                List<T> toRemove = new List<T>(Count);
                 BreadthFirstTreeWalk(n => { toRemove.Add(n.Item); return true; });
-                while (toRemove.Count != 0)
+
+                for (int i = toRemove.Count - 1; i >= 0; i--)
                 {
-                    _underlying.Remove(toRemove[^1]);
-                    toRemove.RemoveAt(toRemove.Count - 1);
+                    _underlying.Remove(toRemove[i]);
                 }
 
                 root = null;