feat(content): more test cases

backend/data/questions/invert-binary-tree.yaml

@@ -11,6 +11,24 @@ patterns:
   - dfs
   - bfs
 
+function_signature: "def invert_tree(root: TreeNode | None) -> TreeNode | None:"
+
+test_cases:
+  visible:
+    - input: { root: [4, 2, 7, 1, 3, 6, 9] }
+      expected: [4, 7, 2, 9, 6, 3, 1]
+    - input: { root: [2, 1, 3] }
+      expected: [2, 3, 1]
+    - input: { root: [] }
+      expected: []
+  hidden:
+    - input: { root: [1] }
+      expected: [1]
+    - input: { root: [1, 2] }
+      expected: [1, null, 2]
+    - input: { root: [1, null, 2] }
+      expected: [1, 2]
+
 description: |
   Given the `root` of a binary tree, invert the tree, and return *its root*.