feat(content): hidden test cases

backend/data/questions/serialize-and-deserialize-binary-tree.yaml

@@ -11,6 +11,24 @@ patterns:
   - dfs
   - tree-traversal
 
+function_signature: "class Codec"
+
+test_cases:
+  visible:
+    - input: { root: [1, 2, 3, null, null, 4, 5] }
+      expected: [1, 2, 3, null, null, 4, 5]
+    - input: { root: [] }
+      expected: []
+  hidden:
+    - input: { root: [1] }
+      expected: [1]
+    - input: { root: [1, 2] }
+      expected: [1, 2]
+    - input: { root: [1, null, 2] }
+      expected: [1, null, 2]
+    - input: { root: [1, 2, 3, 4, 5, 6, 7] }
+      expected: [1, 2, 3, 4, 5, 6, 7]
+
 description: |
   Serialization is the process of converting a data structure or object into a sequence of bits so that it can be stored in a file or memory buffer, or transmitted across a network connection link to be reconstructed later in the same or another computer environment.