feat(content): more test cases

backend/data/questions/binary-tree-level-order-traversal.yaml

@@ -10,6 +10,24 @@ patterns:
   - bfs
   - tree-traversal
 
+function_signature: "def level_order(root: TreeNode | None) -> list[list[int]]:"
+
+test_cases:
+  visible:
+    - input: { root: [3, 9, 20, null, null, 15, 7] }
+      expected: [[3], [9, 20], [15, 7]]
+    - input: { root: [1] }
+      expected: [[1]]
+    - input: { root: [] }
+      expected: []
+  hidden:
+    - input: { root: [1, 2, 3, 4, 5] }
+      expected: [[1], [2, 3], [4, 5]]
+    - input: { root: [1, 2, null, 3] }
+      expected: [[1], [2], [3]]
+    - input: { root: [1, null, 2, null, 3] }
+      expected: [[1], [2], [3]]
+
 description: |
   Given the `root` of a binary tree, return *the level order traversal of its nodes' values*. (i.e., from left to right, level by level).