feat(content): hidden test cases

backend/data/questions/palindromic-substrings.yaml

@@ -10,6 +10,26 @@ patterns:
   - two-pointers
   - dynamic-programming
 
+function_signature: "def count_substrings(s: str) -> int:"
+
+test_cases:
+  visible:
+    - input: { s: "abc" }
+      expected: 3
+    - input: { s: "aaa" }
+      expected: 6
+  hidden:
+    - input: { s: "a" }
+      expected: 1
+    - input: { s: "aa" }
+      expected: 3
+    - input: { s: "ab" }
+      expected: 2
+    - input: { s: "aba" }
+      expected: 4
+    - input: { s: "abba" }
+      expected: 6
+
 description: |
   Given a string `s`, return *the number of **palindromic substrings** in it*.