feat: Add solution for LeetCode problem 226

Author: WhiteHyun

invert-binary-tree/WhiteHyun.swift

@@ -0,0 +1,23 @@
+//
+//  226. Invert Binary Tree.swift
+//  https://leetcode.com/problems/invert-binary-tree/description/
+//  Algorithm
+//
+//  Created by 홍승현 on 2024/05/04.
+//
+
+import Foundation
+
+final class LeetCode226 {
+  func invertTree(_ node: TreeNode?) -> TreeNode? {
+    guard let node else { return nil }
+
+    let left = node.left
+    let right = node.right
+
+    node.left = invertTree(right)
+    node.right = invertTree(left)
+
+    return node
+  }
+}