feat(content): more test cases

backend/data/questions/letter-combinations-of-a-phone-number.yaml

@@ -10,6 +10,22 @@ categories:
 patterns:
   - backtracking
 
+function_signature: "def letter_combinations(digits: str) -> list[str]:"
+
+test_cases:
+  visible:
+    - input: { digits: "23" }
+      expected: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]
+    - input: { digits: "" }
+      expected: []
+    - input: { digits: "2" }
+      expected: ["a", "b", "c"]
+  hidden:
+    - input: { digits: "7" }
+      expected: ["p", "q", "r", "s"]
+    - input: { digits: "79" }
+      expected: ["pw", "px", "py", "pz", "qw", "qx", "qy", "qz", "rw", "rx", "ry", "rz", "sw", "sx", "sy", "sz"]
+
 description: |
   Given a string containing digits from `2-9` inclusive, return all possible letter combinations that the number could represent. Return the answer in **any order**.