docs(docs): add Approach diagrams and Explanation sections to LeetCode problem documentation

apps/docs/content/(tree)/226-invert-binary-tree.mdx

@@ -25,6 +25,17 @@ sidebar:
 - The number of nodes in the tree is in the range [0, 100].
 - `-100 <= Node.val <= 100`
 
+## Approach
+
+```mermaid
+flowchart TD
+  S(["invertTree(root)"]) --> B{"root is None?"}
+  B -- yes --> Z(["return None"])
+  B -- no --> L["left = invertTree(root.left), right = invertTree(root.right) — both subtrees are already inverted"]
+  L --> X["root.left = right, root.right = left"]
+  X --> E(["return root"])
+```
+
 ## Solution
 
 ```py
@@ -47,3 +58,7 @@ class Solution:
 
         return root
 ```
+
+## Explanation
+
+<YouTube id="OnSn2XEQ4MY" title="Invert Binary Tree - Depth First Search - Leetcode 226" />