docs: rename category folders to remove parentheses and update heap meta title

apps/docs/content/tree-dfs/226-invert-binary-tree.mdx

@@ -0,0 +1,64 @@
+---
+title: '226. Invert Binary Tree'
+description: Given the root of a binary tree, invert the tree, and return its root
+sidebar:
+  label: 'Invert Binary Tree'
+  badge: 'Easy'
+---
+
+<Badge variant="accent">Tree</Badge>
+
+### Example 1:
+- Input: `root = [4,2,7,1,3,6,9]`
+- Output: `[4,7,2,9,6,3,1]`
+
+### Example 2:
+- Input: `root = [2,1,3]`
+- Output: `[2,3,1]`
+
+### Example 3:
+- Input: `root = []`
+- Output: `[]`
+
+### Constraints:
+
+- 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
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def invertTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
+        if not root:
+            return None
+
+        left = self.invertTree(root.left)
+        right = self.invertTree(root.right)
+
+        root.left = right
+        root.right = left
+
+        return root
+```
+
+## Explanation
+
+<YouTube id="OnSn2XEQ4MY" title="Invert Binary Tree - Depth First Search - Leetcode 226" />