Add cpp unordered set reverse

content/cpp/concepts/unordered-set/terms/reserve/reserve.md

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+---
+Title: 'reserve()'
+Description: 'Requests capacity change for `std::unordered_set` so it can accommodate at least n elements without exceeding the maximum load factor.'
+Subjects:
+  - 'Code Foundations'
+  - 'Computer Science'
+Tags:
+  - 'Containers'
+  - 'Sets'
+  - 'STL'
+CatalogContent:
+  - 'learn-c-plus-plus'
+  - 'paths/computer-science'
+---
+
+The **`reserve()`** method requests a capacity change for an [`std::unordered_set`](https://www.codecademy.com/resources/docs/cpp/unordered-set). It sets the number of buckets to the amount needed to accommodate at least `n` elements without exceeding the container’s `max_load_factor()`. Calling `reserve(n)` may trigger a rehash; if it does, all iterators are invalidated, but references and pointers to elements remain valid. This operation does not sort or otherwise order elements.
+
+## Syntax
+
+```pseudo
+unordered_set_name.reserve(n);
+```
+
+**Parameters:**
+
+- `n` (size_type): Minimum number of elements the container should be able to accommodate without exceeding `max_load_factor()`.
+
+**Return value:**
+
+This method doesn't return anything (`void`).
+
+## Example
+
+This example shows bucket capacity before and after `reserve()`, and demonstrates that elements remain accessible with the same unordered iteration semantics:
+
+```cpp
+#include <iostream>
+#include <unordered_set>
+using namespace std;
+
+int main() {
+  unordered_set<int> numbers = {10, 20, 30};
+
+  cout << "Buckets before: " << numbers.bucket_count() << "\n";
+
+  numbers.reserve(10);
+
+  cout << "Buckets after: " << numbers.bucket_count() << "\n";
+  cout << "Max load factor: " << numbers.max_load_factor() << "\n";
+  cout << "Load factor: " << numbers.load_factor() << "\n";
+
+  for (int i = 1; i <= 7; ++i) {
+    numbers.insert(i * 5);
+  }
+
+  cout << "Size: " << numbers.size() << "\n";
+  cout << "Buckets final: " << numbers.bucket_count() << "\n";
+
+  return 0;
+}
+```
+
+The output of this code is:
+
+```shell
+Buckets before: 3
+Buckets after: 11
+Max load factor: 1
+Load factor: 0.272727
+Size: 7
+Buckets final: 11
+```
+
+## Codebyte Example
+
+This example demonstrates `reserve()` with strings, and prints capacity-related information:
+
+```codebyte/cpp
+#include <iostream>
+#include <string>
+#include <unordered_set>
+using namespace std;
+
+int main() {
+  unordered_set<string> words = {"alpha", "beta", "gamma"};
+
+  cout << "Before: buckets=" << words.bucket_count() << ", size=" << words.size() << "\n";
+
+  words.reserve(10);
+
+  cout << "After: buckets=" << words.bucket_count() << ", size=" << words.size() << "\n";
+
+  words.insert("delta");
+  words.insert("epsilon");
+  words.insert("zeta");
+
+  cout << "Load factor: " << words.load_factor() << "\n";
+  cout << "Max load factor: " << words.max_load_factor() << "\n";
+
+  for (const auto& w : words) {
+    cout << w << "\n";
+  }
+
+  return 0;
+}
+```