feat(content): function signatures + test cases

backend/data/questions/balance-a-binary-search-tree.yaml

@@ -9,6 +9,26 @@ patterns:
   - tree-traversal
   - dfs
 
+function_signature: "def balance_bst(root: TreeNode) -> TreeNode:"
+
+test_cases:
+  visible:
+    - input: { root: [1, null, 2, null, 3, null, 4] }
+      expected: [2, 1, 3, null, null, null, 4]
+    - input: { root: [2, 1, 3] }
+      expected: [2, 1, 3]
+  hidden:
+    - input: { root: [1] }
+      expected: [1]
+    - input: { root: [1, null, 2] }
+      expected: [1, null, 2]
+    - input: { root: [3, 2, null, 1] }
+      expected: [2, 1, 3]
+    - input: { root: [1, null, 2, null, 3, null, 4, null, 5] }
+      expected: [3, 1, 4, null, 2, null, 5]
+    - input: { root: [5, 4, null, 3, null, 2, null, 1] }
+      expected: [3, 1, 4, null, 2, null, 5]
+
 description: |
   Given the `root` of a binary search tree, return *a **balanced** binary search tree with the same node values*. If there is more than one answer, return **any of them**.