feat(content): hidden test cases

backend/data/questions/building-h2o.yaml

@@ -8,6 +8,28 @@ categories:
 patterns:
   - synchronization
 
+function_signature: "class H2O"
+
+test_cases:
+  visible:
+    - input: { water: "HOH" }
+      expected: "HHO"
+    - input: { water: "OOHHHH" }
+      expected: "HHOHHO"
+  hidden:
+    - input: { water: "HHOOHH" }
+      expected: "HHOOHH"
+    - input: { water: "HOHOHOHOHOHO" }
+      expected: "HHOHHOHHOHHO"
+    - input: { water: "HHO" }
+      expected: "HHO"
+    - input: { water: "OHH" }
+      expected: "HHO"
+    - input: { water: "OOOHHHHHH" }
+      expected: "HHOHHOHHO"
+    - input: { water: "HHHHHHOOO" }
+      expected: "HHOHHOHHO"
+
 description: |
   There are two kinds of threads: `oxygen` and `hydrogen`. Your goal is to group these threads to form water molecules.