feat(content): function signatures + test cases

backend/data/questions/average-of-levels-in-binary-tree.yaml

@@ -10,6 +10,24 @@ patterns:
   - bfs
   - tree-traversal
 
+function_signature: "def average_of_levels(root: TreeNode) -> list[float]:"
+
+test_cases:
+  visible:
+    - input: { root: [3, 9, 20, null, null, 15, 7] }
+      expected: [3.0, 14.5, 11.0]
+    - input: { root: [3, 9, 20, 15, 7] }
+      expected: [3.0, 14.5, 11.0]
+  hidden:
+    - input: { root: [1] }
+      expected: [1.0]
+    - input: { root: [1, 2, 3, 4, 5, 6, 7] }
+      expected: [1.0, 2.5, 5.5]
+    - input: { root: [1, 2, null, 3, null, 4] }
+      expected: [1.0, 2.0, 3.0, 4.0]
+    - input: { root: [0, -1, 1] }
+      expected: [0.0, 0.0]
+
 description: |
   Given the `root` of a binary tree, return *the average value of the nodes on each level in the form of an array*.