feat(content): hidden test cases

backend/data/questions/check-if-word-is-valid-after-substitutions.yaml

@@ -9,6 +9,30 @@ categories:
 patterns:
   - monotonic-stack
 
+function_signature: "def is_valid(s: str) -> bool:"
+
+test_cases:
+  visible:
+    - input: { s: "aabcbc" }
+      expected: true
+    - input: { s: "abcabcababcc" }
+      expected: true
+    - input: { s: "abccba" }
+      expected: false
+  hidden:
+    - input: { s: "abc" }
+      expected: true
+    - input: { s: "a" }
+      expected: false
+    - input: { s: "ab" }
+      expected: false
+    - input: { s: "abcabc" }
+      expected: true
+    - input: { s: "aabbcc" }
+      expected: false
+    - input: { s: "abcabcabc" }
+      expected: true
+
 description: |
   Given a string `s`, determine if it is **valid**.